Family Dilemma

So yesterday I got in a huge argument with my sister and it's been built up for some time now. There are multiple things are don't see eye to eye on and we are currently about to move in together for another year so I am a bit worried.

1. I am a first time mom with a 10 month old and my partner isn't comfortable having her watch our son alone. I've explained to her that he feels protective and just doesn't know how mature she is. He is worried that she might not pay attention to him and he could injuried. I did have her watch him in the past when I needed to go to work and didnt tell my partner beacuse I didnt really have a probelm with her watching him. I just assumed he would be okay with it until I talked to him and learned he wasn't. After that time she also has watched him while we were here, but he has fallen and hit his head pretty bad and also would do stuff like give him his passifer after it fell on the floor without washing it off. When I did address it she had an attitude about it. She explained she feels her age 20 shouldn't be taken into consideration when choosing to not let her watch him alone and she feels it's only against my side of the family. She also feels like I don't speak up and just let my partner make the decisions about our son. During our argument she was telling him I was hiding that she watched our son which made me feel uncomfortable as hell even though my partner and i talked about it. I know her feelings are hurt, but I don't understand why she can't be respectful about our decision or why she feels entitled to know what conversatiom took place.

2. She's upset because my bf and her aren't super close. Which I understand to an extent, but I feel like relationships take time. She feels like he treats her like shit because they don't say hi to each other some times or have deep conversations. I understand she wants to have a more intimate relationship, but sometimes it feels kinda weird to me as it's like almost a relationship desire you would have with your partner not a brother. In addition to this, she really wears like no clothes at our house like tiny shorts see through shirts. I don't know I just feel like the behavior and desire is weird asfck. 😕

I do want to make sure I'm walking into a healthy setting for myself and my son. Does anyone have advice on boundaries or can tell if I'm tripping? Like I don't know maybe I'm wrong in the situation and I'm not being fair.