Managing Your Parents also links here for some stupid reason; and we're apparently not capable of editing the Epic Manual, for some stupid reason. You can find Managing Your Parents here, however. Not that it has any material yet.
Oh, the joy.
Since there does not seem to be anybody willing or knowledgeable on siblings to add anything to this page, I'll add my own experience. Perhaps people will follow suit and when we have enough, somebody can come along and clean it up and make generalized assumptions and data for us.
I grew up with a brother, six years older than I am. Since he was six years older, he was also six years bigger than I was. This led to him teasing and bullying me throughout my childhood with me having no way to resist. However, I was close to him in that I did not talk to anybody in my family except for him. From my mom's perspective, I was quiet and hid things from her and frequently got into trouble at school. From my brother's perspective, I was hyperactive and talked and played a lot.
When I was sixteen or so, my brother left to go to the army. My mother was upset, and I developed my Fe pretty early, attempting to comfort her. I became more outgoing to my family since he left. I escaped from my shell, perhaps.
When he came back from the army, and before he left to live on his own, I had developed in a way that when he did try to continue bullying me, I smashed a good fist across his face, something I had only thought about doing before, perhaps because I felt oppressed by him until he left.
I'm not sure if this helps anything, but it's a good start to this page; maybe someone can find this contribution helpful somehow.
I have a younger sister about six and a half years younger than me. She understands me pretty well and has flat out told me, "you lack empathy" so I've tried to develop it over the years. I understand emotions in a very processed manner so I've come to figure out how to get the desired outcome based on watching other people. The good thing about ISTPs in this regard is we come off as very genuine. Tactfulness with a blunt affect is refreshing for many types of personalities.
Anyway my sister and I get along great and I've raised her quite a bit growing up. She is more responsible than me but I feel lacks in traits I'm strong in. Whereas I lack traits she is strong in. We have compensated in different ways from out interactions.
My experience is quite similar in some regards. I was bullied by my brother too and i also did not defend myself until the moment i snaped his arm. And even during that time it just felt like something i had to do since long ago, like a chorus. Before that, my best method of survival was looking at what made my brother happy and repeating those experiences for him so that he wouldnt unload his frustration on me. Even so, we are very close and there is no bad blood between us anymore -id probably beat him anyways-.
I also was very quiet and hide things, to the point my mom took me to the child psychologist once. I stopped going there after two weeks because they said i was perfectly fine. I used to lie a lot though, i dont know if it is common behaviour between our kind, but it was a method to defend myself. Using different lies i judged reactions from people and acted accordingly.
My little sister seems to love me too, thought i expect her to grow out of it after adolescence.