Hi, everyone!
I had a phase a few years ago when I tried to get as many charts of twins as I could and treat myself with, what I thought was going to be both educational /constructive and fun project. However, I soon had to abandon the whole idea (or to focus only on that and toss away all other charts) because, not only was it an exhausting project but it was also an almost impossible one. Regardless of the fact that all charts I collected belonged to people whom I knew, when it came to this problematic (similar charts, different influences), I figured that i needed much more than to just know the person for 15 years. 95% answers that were necessary for such an adventure were still missing. I'd have to ask these people at least a zillion more questions in order to make a sound conclusions about a single aspect.
A similar (although, almost diametrically opposite) thing happens with charts of siblings, as you pointed out- different charts, same family. What now?
As waybread has already rightly pointed out - the keyword is often "experience". To this, I could add another variation of the same word- "experiental". As someone once said- the reality is only the state of things as they appear to us

. The charts of two siblings will inevitably show them as two different entities who are most likely (or definitely) to experience the same phenomenon in a different manner. To a certain extent, this applies even to twins born only minutes apart. Obviously, we also have to consider the fact that the same potential does not necessarily evolve equally in two different entities and that, whilst one person may use a specific characteristic to its full potential, another one may leave it untouched and allow it to become almost extinct. Unless she actually turns it into a frustrating obstacle.
So, how do we apply this to things that seem solid and unchangeable?! In my opinion, by simply following the good old rule that the reality is how we perceive things and not how the things really are (because, who is to tell that green is really green, after all?!). The same house (the 4th), may look like a castle to one sibling and make her stay in it forever whilst the other one may spend her youth feeling as though she was born in an ugly dungeon. Pretty much the same applies to their relationship with their parents. I have a nice example in my family - my grand aunt was the kind of person that no emancipated woman wants to be. At times, I think that the feminist movement was founded just to punish my grand aunt and ban her actions.

She never worked anywhere, her home was her temple, the kitchen was her altar and the husband was her god. And she wasn't a cold mother either. My two aunts, her daughters, grew up with a loving mother, with parents who weren't horribly strict and had a very happy childhood. However, the mother (much later) confessed that she never enjoyed sex or cuddling whilst her husband, didn't mind peeking under other women's skirts. They didn't talk about it, he had never confessed a single affair but, instinctively, she knew about all of them and, again, she didn't really mind - he was her god, she had her temple and, let's face it, she wasn't willing to give him what he was looking for anyway.
But, when the daughters became women themselves, the differences in how they perceived this relationship became more and more visible. One them saw her parents as the ultimate example of a loving couple and entered in a, more or less, same kind of marriage herself. The other one got married at the age of 25 and, by the time she was pregnant with her 2nd child and only 30, she found out that her husband had been cheating on her. She instantly decided to file for a divorce although she was 7 months pregnant at the time. However, her mother's attempts to make her change her mind only managed to dig a giant well of objections and bitterness that went beyond just her own marriage. Instead of to listen to her advice and either accept it or say that staying with this man would make her a miserable woman for ever and ever, she shifted the focus onto her mother's marriage and tried to make her understand that the life she witnessed in her childhood, that of her mother, was, in her eyes, a life of a miserable and neglected woman. They entered into a long lasting fight that was never fully resolved but, to everyone's great disappointment, this episode also started a long lasting war between her and her sister as the other was was fully supportive of their mother. Up until then, they were very, very close. But, ever since that moment, they have been almost strangers to each other. Whilst one of them cannot stand the idea of a woman living the life once chosen by her mother, the other one actually wants a life of that kind even for herself. Obviously, the 4th house issues are not perceived the same way.
Finally, we sometimes have to assume that the circumstances change as people change. My sister is almost a decade younger than me and, when you look at our charts, you can't but to notice that our childhood was bound to be significantly different. I was a clear example of a spoilt kid, growing up with two loving parents and having everything a kid could ask for. My sister, on the other hand, has a few strong signs of growing up in drastically different circumstances. When I was 17, my father decided to leave his old job and go abroad "for a while". At the time, my mother didn't think that the whole family should move since she had had a steady and well paid job and loved her town. besides, it was only a temporary solution. I moved out soon after that and it didn't really affect my growing up but, since my father remained abroad for almost 2 decades after that, my sister, who was only 7 when he left, basically grew up with our mother. Our grandparents also died in the meantime and the comfort of having a huge, loving family was, in the aftermath, only my privilege. And this goes beyond just "reality" - I never really felt comfortable in my hometown whilst my sister decided to shift her priorities and study economics instead of going to a law school because she didn't want to live anywhere else. Our home town simply looks like a much better place to her than it does to me.
Cheers,
aglaya