Maybe this doesn't fit here... oh well. I'll try.
Generally, I would say that I'm pretty mature. Sure, I like to have fun with my little sister (she's 15) and be a bit immature, but I'm still on a level I'd deem acceptable for a 20 year old. IF I'd be considered immature, it's nothing out of the ordinary.
Now, my big sister left the family in 2016 to live abroad for a year. When she came home, she moved in with her boyfriend and she hasn't been around a lot since. She is one year older than I am. When we were younger, she used to be very mean to me. I'd say she straight up bullied me between the ages of 5-16. I had to be close to her because we shared the same room most of the time but she used to hit me and say very hurtful things to me ("you're worthless", "you're ugly", "you're fat", "you don't deserve to live", "you should kill yourself" - the last one, she said when I was severely depressed and seriously contemplated committing suicide when I was 16 - etc. She said these kinds of things every single day).
She made me her "slave" - I read in one of my old journals yesterday and I wrote something like "I'm not at school today because I'm sick. I'm in the sofa watching the Vampire Diaries. Now my sister is telling me to make her a sandwich even though she knows that I'm sick and she's not.
" I did a LOT of things for her because I was afraid that there would be consequences if I didn't. She threatened me with a lot of things like spitting on my pillow or things like that. She made fun of my self harm and threatened me with a knife. I was afraid of her but at the same time, I became very dependent on her as well and for a while, she was the only friend I had. I was jealous when she was with her friends because I had only her. It was a pretty ****ed up situation, really.
She came home today, and I instantly became very immature. Maybe on the level of a 12-13 year old. I can see on her that she gets annoyed with me, but I can't help myself. It's as though I literally become a 12 year old. I think it's really good that she went abroad because I could finally break free and become my own person. I also become very defensive of my own things - I don't want her to be near my things because i don't trust that she won't look through them... and now she's sleeping in my room. ;/
Anyway, do you think my current immaturity around my sister is a result of the torment I had to face from her during my childhood? Maybe they have nothing to do with each other, but it's just a thought that hit me today as it happened. I even pointed that out to my sister and she was like "but you're always like this" and I was like "yes, with you!" My mother agreed that I do "change personality" and become more immature when I'm around my big sister.
TL;DR: Is the bullying that my sister put me through between the ages of 5-16 the reason why I act like a 12 year old around her even though I'm 20?