What should I do?

Sorry to hear about your situation, I hope things get better for you. Ultimately it's not about your father's feelings it's about you. Your stepmother is not your mum and you clearly dislike her- you want to visit you mum because she is your mum - end of argument. It is always good to keep ties with your family members and maintain good relationships that you can keep in the future also, you need to be honest with your father and tell him how you dislike stepmum's treatment towards you and your sister - therefore wanting to keep distance for your own mental health - it's important that your stepmum's behaviour is dealt with because this can create a rift between you and your half - siblings and will be damaging to the family as a whole tbh. If you don't see your mum now it can cause relationship issues in future which will also be damaging, so it's good to keep that door open.

This might be a little long and confusing so I apologize if it is.
My parents got divorced when I was about 6 or 7 years old at first after the divorce my mom had custody of me and my older sister while my dad had visitations for the weekends and July of summer vacation but due to some issues with my mom CPS got involved and my dad got custody. I forgot to mention that my dad started dating my stepmom a couple months after the divorce, at first I kinda liked my stepmom but after she had my half siblings she changed. She treated my sister like **** and they constantly fought which was a problem to me because my older sister and I were very close so when they fought I would also fight with my stepmom. I think Am(stepmom) hated my sister because she is a lot like my mother they walk, talk, act, and have the same attitude which I guess reminded Am of the relationship between my mom and dad before her. As the years went on the fights between Mm(sister) and Am got worse due to these fights I started having panic attacks but i always had to stop their fights which almost always got physical. Now then my dad had full custody of Mm and I while Sh(bio mom) had weekend visits but due to some not too recent events (last sumner vacay) I have to had decide on supervised visits now the month that Sh is supposed to have me is coming up and I want to see her more often but my dad has started asking me why I would even want to she her at all, I don't know what to tell him i enjoy seeing my mom it gives me a break from dealing with Am's bs but i don't want to hurt my dads feelings what should i do?
If you want to know anything more specific please ask I don't mind sharing I just didn't want to put to much here