AITAH for constantly starting arguments with my older sister over the past and sticking up for our mum?

Me (17f) and my sister (21f) have always had a rocky relationship, especially growing up. When we were younger, I had to bottle up a lot both at school and at home. I couldn’t really be myself because one small mistake and she’d flip. I’d go to school with bruises sometimes. She’d sit on me till I turned blue and purple. Occasionally, we’d have “nice” days out, but it usually involved her trauma-dumping on me (keep in mind I was only 10 and she was 14) and then telling me I couldn’t tell anyone.

I used to give her my birthday money to buy her sweets just so she’d be happy with me. Sometimes I’d even pay her to hang out with me, which I know sounds sad, but that was my normal. Meanwhile, she got whatever she wanted. Hated school? She got homeschooled. Wanted therapy? Mum got her therapy. But me? I got her old clothes that never fit right and were always too small since I was the chubby kid she loved to make fun of. And any time I wanted to sing or act (my biggest passions), she'd mock me.

Now fast-forward to now — she's suddenly all over our dad, hanging out with him constantly, even though he’s been in and out of our lives for years. He’s involved in dodgy stuff, always has a new girlfriend, and barely checks in unless he wants to show us off to impress them. It hurts my mum, and it hurts my aunt (dad’s sister) who basically helped raise us through all that drama. But my sister treats our mum like dirt now. I get that she’s trying to get back the years she lost with our dad — I probably would too — but it’s just hard to watch, especially knowing what he’s like and how much it’s affecting my mum.

Now my mum’s pregnant again, second time since having me and my sister. She nearly died last time during birth and had to have an emergency C-section. This time, she’s got diabetes and is on a bunch of meds, and it’s really scary — she could collapse at any time. I'm a chronic overthinker now because of it. When I try to talk to my sister about it or when she comes to visit, she says stuff like “Mum brought this on herself” or “She deserves it.” It's cold, and honestly it makes me angry.

Last time I spoke to her, I stood up for my mum, and it blew up into a huge fight. She switches so quickly, and it’s like I’m always walking on eggshells with her — even now, years later. I'm just trying to keep some kind of family connection alive, especially between her and my mum. But I’m the one who ends up arguing with her and feeling like the bad guy.

So... AITAH for standing up for my mum and arguing with my sister? Or should I just let it go?