I completely understand where you’re coming from, and the truth is, you can only help someone if they want to be helped.
However, I think the point that many posters, including me are trying to make, is that you don’t know whether she does or doesn’t until you try.
You’ve already said this woman suffers from anxiety and is quite timid, so from her perspective, she’s maybe thinking that everyone finds this funny, so she can’t say anything, and maybe just pushes her own feelings down to save rocking the boat.
Of course, it could be that she genuinely doesn’t mind (which is concerning in and of itself), but there’s nothing much you can do about that.
I would definitely go in and comment with something along the lines of “I hope nobody ever speaks to your daughters like that.”
At the end of the day, you’re already the black sheep, and having less contact with this family is hardly a loss, so it’s better to speak up and perhaps give this woman some support and courage to stop accepting this level of disrespect, rather than just stay quiet and allow them to think everyone is going to put up with it.
I would also be having a serious conversation with your husband about this. I know you said he’s autistic and doesn’t get it, but that’s really not an excuse. He needs to be made aware of the fact that these things are not acceptable to you, and why they’re not acceptable.
If he doesn’t understand the tone of such things, how will he understand when someone decides to speak to or about his daughters in the same way?