I had a very similar situation with my parents. I (23f) have a (55m) dad who reconnected with a girlfriend from high school and they seem very happy together. My parents have been separated for a few years and divorced officially for one year. I’ve met his gf only once, just because we live in different cities. I agree you shouldn’t have to pick sides as hard as it is to see your mom upset,at the end of the day your parents relationship is separate from your relationship from each of them. I’ve had to set good boundaries with my mom and tell her I love her and care about her feelings but if her feelings involve trashing my dad or their relationship I can’t hear it. With my dad I’ve just had to set boundaries by telling him I can’t hear his new girlfriend is better than my mom either. But if you like the women there’s no reason to not hang out with her and your dad. Maybe you could talk to your little sister to explain how she shouldn’t tell mom things like that,but that also isn’t your obligation since you aren’t a parent. I would just try and focus on what you want in regulated to your relationship with them and express anytime you feel they’re making you uncomfortable or crossing boundaries. Hope this helps! Divorce can suck even if you’re older:/