I do that as well, it's far easier for me to figure out which part of me works best in a particular environment and then play into that role, rather than just being the same person all the time and making others make room for me.I bring out different aspects of myself, in order to target the group I'm interacting with.....I'm very 'go with the flow' in that regard.
That tends potentially to be an almost Fe vs Fi thing in terms of initial instincts, at least in terms of the social considerations.
I had only one sibling, but she was six years behind me in school and different in personality so we did not interact that much. The family issues really disrupted normal social engagement. My alcoholic dad was also ESTP, the only extrovert in the family, and he was insensitive, loud, abrasive, and overbearing when he was around... there wasn't much for all the introverts to do but withdraw / avoid conflict because we could never get him to change his mind or approach things differently. Avoidance was easiest, but my second developed relational practice was compliance and/or figuring out what someone wanted and giving it to them, not making waves (no matter how I felt inside), because conflict could not be won and therefore needed to be avoided if I was to retain maximum autonomy.
This was something I had to get over as an adult, realizing I had power in a negotiation and thus I could engage honestly and effectively rather than just in ways to reduce hostilities.
Anyway, my sib and I really did not much impact each other's social dev, the situation with Dad was far overshadowing. I think only-children do get different dynamics from sib-dynamics, though, and even parental attitudes and attentions.