I can hear how hard it is for you, Carinacan. In my experience there are many, many different reasons why a parent may not be able to admit their child experienced abuse, let alone be able to give them support. In my own family my mother knew what was going on in the family home but never did anything about it. As a child I believed it was because of I deserved it / didn't matter / was imagining it... all sorts of different things. When I confronted my mother years later she told me the real reason she did nothing about it: She said if she had believed it or allowed herself to think about it she wouldn't be able to bear sleeping with my father ever again. So... my siblings and I continued to suffer years upon years of abuse so my mother could continue to have a relationship with my father.
That scenario is far from uncommon. Parents have their own reasons for not believing or protecting their children, and what it mostly comes down to for those parents is that their primary need is to protect themselves. Maybe they don't have the internal resources to cope with dealing with it, maybe they are unable to tolerate conflict, maybe facing up to the reality of abuse with cause too much internal pain / guilt / shame or whatever. No matter what the reason is in any given family, the end result is the same - the parent fails to protect or believe the child because of their OWN inadequacies and inability to deal with what is happening.
Your mother may never be able to face up to this issue. My own mother still can't. All I can say to is... your mother's inabililty to support you is not about you. It's about her stuff, her inadequacy, her inability to cope.
On the bright side, YOU can stick up for you. If she wants you to go there you can stick up for yourself quietly but firmly. "I'm sorry mom. I don't want to go there and you already know the reasons why." If she rants and yells, calmly say something like "I'm really sorry you don't believe me mom, but I will not visit them." Stay calm. You have already told her why you don't want to go there - yelling it at her all over again isn't going to make her change her mind. She knows the reaons, and she gets upset because she doesn't want it to be true as she can't deal with it. You can'tn change her, but you can be true to you.