I don't think people can assume that because the father was violent to the mother, that he would be abusive towards the children after she left.
Presumably she knew the extent of the abuse best, and thought they would actually be happier without interaction between the parents which would be necessary to share custody, and which is often a trigger for continued abuse (therefore affecting the children).
He might well have treated them OK without her around. My ex was abusive to me, and still finds opportunities to manipulate me via our daughter. But if she was with him full time, that dynamic would no longer exist, and despite me disagreeing with his parenting methods, I think he would be a good enough parent in that case.
Yes he's sexist, but so is the rest of society, so I don't think he would cause her to absorb those attitudes any extra than she would anyway (especially since I've done a lot of anti-sexist groundwork already). It wouldn't be considered to be on the level of abuse anyway (if anything SS are very conventional in that regard, and expect to see girls wearing pink and having long hair).
Never mind the other issues with judgement from society at that time, lack of a suitable home/job/resources etc.
It must have been a terrible wrench for her to leave them; I doubt she was the cold-hearted person other posters are making her out to be. She thought she was taking the least bad option.
The fact that she met someone else is irrelevant, other than it maybe giving her the motivation to leave in the first place. Would people be judging her so much if she had left without there being another man involved? But the situation would otherwise be the same.
So I don't she she IBU to not see them, especially in her current state and after so long. She will be easily distressed, especially if she feels she has to defend herself, and they all get angry with her. And she may not be able to explain what happened coherently anyway.
Also YANBU for not wanting to see them, especially if they are homophobic. And they may not want to hear that their dad was abusive, so may get defensive about that. Is he still alive? If not, maybe that is a good sign, i.e. they didn't want to contact you while he was alive, as they felt disloyal, but maybe they do know or suspect something of what went on, and are now open to hearing it.
What about if you sat down with your mum and wrote a letter or took notes about the situation, so they can have an explanation, and you could either send them that or just meet them once to talk to them about it, but not have an ongoing relationship?