AITA for lashing out at my sister and turning my brother against her?

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The original was posted on /r/AmItheAsshole by /u/throwawaydadskid on 2023-06-27 05:38:12+00:00.

My (15f) Dad (38)had my oldest sister Bella (22) when he was 15 years old with her mother Satan (41). It was a traumatic time for him as Satan left the country pregnant and Dad followed her over, they broke up when he caught her cheating on him and Dad wasn't allowed to see my sister until she was 10 after my Mum (37) had been speaking to Satan for 4 years to try and establish a relationship.

I was 2 when I first met her after my Mum payed for her to come over to our country with her mum to meet us. My brother (14 now 8 months then) was in hospital when they came over as he nearly died the week before they were coming here from SIDS, Dad did CPR, he came out the day they arrived.

For relevance my mum had my two other sisters when her and my dad got together 19 years ago but Dad took them both on & Mum, my Dad and sisters Dad have always gotten along and their Dad still comes for dinners and I call him Uncle.

Anyway, when they came over they both tried to split my mum and dad up and said horrible things about my mum & brother & sister and asked my dad to choose them two or us, Dad chose us and they cut him off again - My sisters and mum were treated horribly by Bella and her Mum the week they stayed.

My sister Bella recently showed up and is living with us for a month now, I don't know why but she shouldn't be here. Mum has gone to stay with her parents as she doesn't want to cause any issues & wants dad and us to spend time with her & Dad has been quiet but my brother loves her. I told my sister she needs to leave because I heard mum saying she's going to leave Dad as it's to traumatic having my sister here to my Aunty & I told my brother everything my sister and her mum done when they were here and now he hates her and wants her to leave too and I told her she's not my sister or my dads kid legally so shouldn't be here.

Mum is upset with us for not trying to build a relationship with Bella and for making it hard for Dad and wants us to apologise to her and forgive her but Bella never apologised to my mum so am I the asshole?