I (17m) might be TA and I might be a big one but I wanted to get people's insight.
So my dad left my mom when I was 14 to be with his wife/the woman he was cheating on my mom with. My dad didn't want me to think badly of him but I did. He knew I did and he knew he couldn't change my mind easily. We always knew my parents would end up with shared custody until I was 17 at least (from experience the judges in family court will only stop enforcing shared custody when a kid turns 17 and speaks out, any younger and they insist on 50/50). I also knew mom would struggle on her own because while she did work, she never made as much as my dad. She also wouldn't get child support because of the 50/50 and it wasn't ordered even with the difference in income. So when dad pleaded with me to give him a chance to show he could still be a good dad and he said he would do anything for me, I told him to keep supporting mom and make sure she wasn't going to end up struggling while he got off easy. I told him she deserved that at least, after what he did, and that I deserved to see my mom doing well.
My dad agreed and he paid it as child support instead of spousal support or whatever it's called. It really helped mom and she actually went back to school so she could get a better job. Dad is still paying that money. He knows I appreciate him doing it and he also knows it's one of the only reasons I didn't just decide to say fuck him and never want a relationship again. My mom is also less stressed. She has mixed feelings about my dad giving her money when legally he doesn't need to. But she also knows this is the only way for me not to feel the need to help support her.
Where my dad's wife comes into it is this. She never liked that my dad paid this "child support". She never liked that I insisted on it for my dad and I to have a relationship. But now my dad's house is struggling a bit and some changes needed to be made. His wife's son and daughter were in dance, football, softball, karate, music lessons and an art class as paid extra curricular's. My dad and his wife also have a baby together. The wife's kids had to cut two activities because they can't afford it anymore. It pisses his wife off because if dad wasn't paying the money they could still afford those things. She told me I should stop obligating my dad to support mom because they need it more and my mom isn't their problem. I told her she and her kids are not my problem. She told me they're my family while mom is not her or her kids' family or my dad's anymore. I told her she and her kids are dad's family but not mine. She told me they need all of dad's money right now before more things need to be cut back on. I shrugged in response. She told me I was so callously flippant and it wasn't a good look to care so little about my siblings lives (only one of her kids, the baby, is technically my half sibling).
AITA?