marl I am sorry you are in this difficult position.
This woman is not just your sister in law she is the mother of your nieces/nephews or your husband's nieces/nephews (depending on how you look at things); so the mother of your children's cousins.
I would personally want to respond to her and make contact. Be a listening ear and advise her just as you would any other woman in this situation.
What you say may well be dependent on where she is geographically but ignoring her plea for help seems a very bad idea. You are not duty bound to defend or cover up for your BIL.
So if she is in the UK you could advise her of her rights under UK law or at least point her to Women's Aid who can do that for her. Talk to your dh and be open about this. But frankly I think Fuck what the other members of your husband's extended family think.
If she is in Thailand (or elsewhere overseas) then you could still encourage her to seek help. There are organisations all around the world which help women and she will be able to Google those wherever she is. But maybe she needs someone to listen first.
I just Googled and got a Thai charity working in Thailand with women.
You can be very clear about your own boundaries, such as not sending or giving money. Has she asked for money? Why are you fearful of that?
I think you do need to protect yourself because of your own past so I might be tempted to share this with a good friend outside the family and get support from them. And talk to your dh. You do not need to involve his sister, she can keep her head in the sand.
I think probably this poor woman just needs someone to talk to, maybe that could be you, while she decides what to do. You can offer that, but can ensure she knows you can't offer money or whatever. If this was me I would talk to DH and decide together what you plan to do.
Your BIL sounds appalling and to be honest your English SIL is the one who sounds nutty, when is "His sister took the line that this was yet another in the line of his 'nutty partners'" ever a good response to a woman who is claiming domestic abuse.