Ginny's Canon Relationship with her Brothers
Let's start with the oldest brother, Bill. Ginny stands up for him when their mother expresses her distaste for his fashion statements, which clearly shows that the large age difference between her and her oldest brother doesn't mean very much to her. She obviously values Bill's opinions of people:
'Bill doesn't like him either,' Ginny said, as though that settled the matter.
In the Christmas of her first year, Ginny gives up the change to go and visit Bill in order to stay at Hogwarts. This could be considered a sign that she doesn't like him all that much, but it's more likely that she was staying because of Tom.
She laughs along with the twins at the sight of Bill and Charlie are levitating tables, when other spectators are more anxious and less amused. She also sports green rosettes like Bill and Charlie at the Quidditch World Cup, showing that she evidently trusts and shares their opinion on Quidditch matters.
Charlie is the first to reassure Ginny during the Quidditch World Cup, when she is looking horror-struck at Krum's Wronski Feignt. It can be considered an unusual reaction for Ginny, who at this age usually matches her brothers in behaviour, but Charlie isn't surprised at all.
We also see Percy comforting Ginny, although this is during her first year when we know she is not her usual self. We hear from Percy that she comes to him crying her eyes out over the trio being accused of Petrifying Mrs Norris, and he admonishes Ron for upsetting her. Ron claims that Percy doesn't care about Ginny and that he's really concerned for his own reputation, an idea that Percy doesn't bother denying. However, Percy later threatens to write and tell Mrs Weasley that Ginny is having nightmares, if the twins don't leave her own, suggesting that he really does care about her.
Ginny confesses to Harry that she wanted to tell him about Tom, but not in front of Percy. This implies that she doesn't trust him, however it also shows that she knows him very well and knows he doesn't believe unlikely stories without solid proof.
Towards the end of Chamber of Secrets, Ginny seems to be a little frightened of Percy. This could be taken to mean that Percy has been harsh with her over keeping his secret about Penelope, but after the events in the Chamber, however, she happily tells the others about Percy and his girlfriend, so we can probably blame her uneasiness on Tom. Despite revealing Percy's secret, however, she asks the twins not to tease Percy, showing that although she giggles about him in private, she doesn't want to hurt his feelings. In Prisoner of Azkaban, we see that Percy considers himself to be a role model for his little sister ("Ginny's got other brothers to set her an example, Mother").
Whilst the others tease Percy about his job, Ginny shows an interest, "Why are they all sending Howlers?". She may or may not do this to make him feel better, but even if she is asking out of genuine interest it shows she considers him to know what he's talking about.
When Percy leaves the family in Order of the Phoenix, Ginny confess that "It's been awful", and Ron repeats what Percy said to their father, Ginny "makes a noise like an angry cat", indicating that she does not sympathise with Percy at all.
Onto the twins. When we first meet Ginny in Philosopher's Stone, she is comforted by the twins, "Don't, Ginny, we'll send you loads of Owls." George even offers to send her a Hogwarts toilet seat. In Chamber of Secrets, we see them making fun of Ginny behind her back, "She'll be wanting your autograph, Harry", which shows that they don't give her any special treatment just because she's a girl. When Ginny is upset over the Petrifications, they attempt to cheer her up by covering themselves in fur and boils. Harry and Percy don't think this is the right way to comfort Ginny, but we don't know what her reaction was.
Ginny doesn't find it funny when the twins tease Harry about the idea that he may be Heir of Slytherin. However, like many of the events of her first year, this could be explained by Tom's effect on her. We may not be able to consider her behaviour of this year to be like her true self.
After Chamber of Secrets, however, we start to see Ginny acting more and more like the twins, but still with a kind mature side. She laughs at their antics, but when they tease Ron about Scabbers, she reprimands them. In Order of the Phoenix, when she and the twins are arguing with Sirius about visiting Arthur, she gives up after hearing Sirius' sensible points, while the twins remain mutinous for a minute longer, and then go to sit with her.
During the riots at the Quidditch World Cup, it's Fred who grabs Ginny's hand as they head for safety, and she stays with the twins until they all meet up at the tent again.
She is impressed by Weasleys' Wizard Wheezes, and perhaps it gives her a new-found respect for them, as she says, "We never thought they were actually making things, we just thought they liked the noise". By the time of The Order of the Phoenix, she is collaborating with them, "It's no go with the Extendable Ears, she's gone and put an Imperturbable Charm on the door." She even teaches Fred and George a thing or two, "You just chuck stuff at the door and if it can't make contact, the door's been Imperturbed." When they hear that Harry has not been expelled after all, Ginny and the twins dance around the kitchen singing, to the obvious annoyance of Mrs. Weasley. She informs them when she finds out Harry wants to talk to Sirius, knowing they can help.
Far from being the shy and uneasy girl she was in Chamber of Secrets, Ginny has become cool and collected, able to lie "unblushingly" to her mother, seemingly under the influence of the twins: "The thing about growing up with Fred and George… is that you sort of star thinking anything's possible if you've got enough nerve." However, she still has a mind of her own - she never tells her brothers that she has been stealing their broomsticks to practice alone since the age of six, which shows that she's always been strong-willed and resourceful, despite appearing otherwise during Chamber of Secrets. When Hermione reveals Ginny's broomstick secret to the twins, they are impressed.
Harry often considers that Ginny looks very much like Fred and George at times, and when the twins leave Hogwarts, Ginny seems to take on their role:
"Luna and I can stand at either end of the corridor," said Ginny, promptly, "and warn people not to go down there because someone's let off a load of Garrotting Gas."
Hermione looked surprised at the readiness with which Ginny had come up with this lie; Ginny shrugged and said, "Fred and George were planning to do it before they left."
At first, Ron doesn't seem to want Ginny to be around them very much. In Chamber of Secrets, Ron realises that Ginny is acting strangely ("You don't know how weird it is for her to be this shy. She never shuts up normally.-" ) but does little about it, perhaps partly due to Percy blaming him for Ginny's odd behaviour, and only takes the whole situation seriously when he discovers she's been taking into the Chamber.
After that, Ron still isn't very nice to her, often telling her to leave them alone. However, she doesn't seem to be very hurt about this, and it's clear that Ron has difficulty being nice to anybody he likes, so this simply means that he doesn't always understand Ginny very well. He probably spent a lot of time being stuck with Ginny just because they were the youngest, and resents that fact. Despite there being little age difference between them, Ginny is often treated differently, something she clearly resents -when Molly allows the rest of them to stay at the meeting and not Ginny, she storms upstairs very loudly, "raging at her mother".
By the time of Goblet of Fire, Ron seems to have relented, and allows her to spend time with them. He clearly doesn't see her crush on Harry as any big deal - when he and Harry need dates for the Yule Ball he casually suggests they go together, not seeming to think anything of it. Similarly, he doesn't mind that she goes to the ball with Neville. It's only when she starts to date Michael Corner than he gets over-protective, perhaps because she didn't tell him, perhaps because Michael is a stranger to him and therefore unpredictable. Later, he's also angry that she is dating Dean Thomas, who has previously appeared to be a good friend of Ron's, signifying that he has very high standards of who is good for Ginny. She knows Ron well enough not to tell him about Michael, but by the time she meets Dean Thomas, she has adapted well enough to their changing relationship to know that the right thing to do is tell Ron bluntly.
Ginny tells Ron off for being cruel about people such as Neville and Luna. He looks to Ginny for explanation as to Luna's odd behaviour, perhaps because of the fact that she pays more attention to people than he does. Ron and Ginny seem to contrast one another in other ways. When Hermione tells Harry that she, Ron and Ginny have been talking about him, Ron looks at his feet but Ginny "seemed quite unabashed" and even speaks up, implying that as what they say is true, they're allowed to say it. It would be easy to imagine, that as the two more important of the Weasley siblings, they would look alike - but Ron is tall and looks more like Percy and Bill, while Ginny is remarkably similar to the twins.
Ginny seems to be becoming more and more like the twins, is able to get along with Bill and Charlie as any adult would, she's had several interesting encounters with Percy, and is able to let Ron know she's not just a little sister. Ultimately, we can see that Ginny is a quick learner and picks up a lot of things from her brothers, whether from their mistakes or their adventures, and they certainly have a big part to play in shaping the woman Ginny Weasley will grow up to be.