The people you are surrounded with have their own issues then.
If my child was treated like this i'd be raging! So would any normal parent.
What would you feel if someone did this to your little girl? Would you tell her not to make a fuss and to just take it?
Your dad sees it's not right, why not her?
Thing is chick, when we get sucked into a relationship with people as damaged as your ex and his awful family, when we haven't wised up to them and booted them to the kerb when big red flags were showing themselves (and you know you had your worries about him ages ago) then something is wrong in the way we see relationships.
Most people (with healthy dynamics) would have never got this far with this guy, they'd have seen the signs and acted on them. You saw signs, but shouted down your instincts down. You didn't want him to be as bad as people pointed him out to be.
That's ok, we've many of us done it, but when we see the truth, we need to face it and deal with it.
Now you know the truth about this terrible man, and your mum still thinks it's 'not that bad' or not bad enough.
What would be bad enough for your mum then? What's bad enough for YOU?
Not critising her, but why is her view of what's acceptable so skewed? Were her parents unhappy?