I think birth order has had a big impact on me. I'm the youngest of 2 and never had a say in anything we did as a family. My DF died young so I think perhaps my older DB's role in the family was elevated so he was half older sibling and half "the man of the house". He had loads of small responsibilities that I never had. Just things like mowing the lawn, doing the navigating while my DM drove the car, asking strangers for directions if we were lost, writing our family annual Christmas letter, etc. I was basically invisible to my DM. I never had to do anything, but I was also never allowed to do anything either. Small things like what to watch on TV would be entirely my mum & brothers decision. I couldn't choose my own food in a restaurant when I was young (my DB wanted to choose for me so e.g. I couldn't have a burger at McDonald's I'd always have to have nuggets, or I'd have to have pasta at an Italian because he didn't want me to have pizza). I also wasn't allowed to choose my own clothes until I was old enough to buy them myself.
I think these things add up over time so I grew to instinctively deprioritise my own feelings. I wasn't really allowed my own opinions (otherwise I would be screamed at and ridiculed) and much of the time I was blanked / ignored if I spoke to DM. Even non-offensive statements like asking my DM how her day had gone would be blanked so I ended up being a very quiet person who didn't talk much. When our pet cat died nobody even told me as I was such an unimportant family member, I found out from a neighbour weeks later (I was sent to boarding school so not at home when it happened). And when I was 15 my DM moved out for a few months because she was fighting too much with DB and again didn't bother to tell me until several days or even weeks (i don't even know) after she'd left and that was only because I had wanted her to pick me up from boarding school so she needed to tell me to get a taxi. She never explained what had happened or bothered to even let me know where she was staying for the time she was away.
I am not sure really whether it was worse for me or my DB. My lack of autonomy and consistently being blanked and ignored resulted in crippling self esteem problems as a teenager. Nearly all my good friends are only children which I don't think is a coincidence as I'm unable to make any decisions or push my preferences on others so I end up being friends with people that don't compromise. I didn't go through a teenage rebellion, instead I turned my feelings inward by self harming and with hindsight had very disordered eating. I still sometimes get very angry as I am prone to feeling powerless which can become anger.
DB went very off the rails as a teenager, I think he was unable to cope with the excessive scrutiny from my DM as she poured everything into him. He was a school bully and got expelled from multiple schools. He has a very odd personality and I've wondered if he's a little bit psychopathic as he never feels guilty and just puts himself first no matter what. He is the polar opposite of me in terms of being extremely outgoing.
Nowadays, none of us are very close and we don't really talk much at all. I grey rocked my DM but she didn't really notice, or if she did she didn't say anything.
This is an interesting topic. I've been thinking about it recently as my passiveness I think is interfering with my ability to be a good parent. I am just rubbish at setting boundaries. I've changed a lot and am so much better than I used to be but these characters traits are so ingrained they never really go away.