Hey Anny, I think it’s a good idea to speak with your husband about this and explore why and what about your contact to his sister would hurt him. If you understand this better then maybe both of you can figure out if any contact to what extent, if so, can be a compromise that makes both of you happy.
And you can also explore just for yourself, what is it in this situation that’s most important to you? Is it important to support your husband? Is it important to maintain the friendship? Is it important to you to do the “morally just” thing” (whatever that may be in this situation)? Do you hope to be able to put an end to their conflict sometime in the future if you stay in contact with his sister?

Try to gain some clarity over what’s important to you and your husband.

I hope this helps. All the best to you.