ESH

This really wasn't easy to arrive at a decision. While others may not agree, the reason I see it this way is because there is a way to be honest without being this level of brutal.

First off, your mom is absolutely an asshole and an enabler for trying to point out that he has changed so you should feel differently about him. That's not how surviving trauma works. You're also allowed to feel poorly about him and not have those feelings invalidated or contested by anyone, especially your mom who clearly understood just how bad what he did while you and your brother were younger based on her response to your "irrelevant" comment.

All of that said, that doesn't enable you to be that level of open with how you feel. While any reasonable person can see she was wrong on a multitude of levels for not having left his ass years ago, she does feel something for him and I assume you love her for your own reasons. Your words clearly hurt her which is a reflection of you and the actions you elected to take and nothing else. It could have just as easily been said that while she may feel that he has changed, that doesn't mean you need to embrace him for several reasons.

Overall, there was a better way to have handled the situation in the moment. But your mom is still the much, much larger asshole in this situation for not leaving him sooner and for giving you push back on why you don't want to embrace him now or into the future.