She has absolutely no right to ask you not to wear a wig, if it's what you usually do to public functions etc. It's not like you're Cher or Lady Gaga, is it. So wear your wig.
Equally, she might not like your fashion sense but so what? Whoever does like all their guests' fashion sense? So no, as you're not part of the wedding party, she doesn't get to choose your clothes for you either.
I really would consider not going to this wedding as it sounds like it'll be a serious trial to you anyway; but I would also be guided by what your DH wants.
If he's prepared to stand up for you and tell his sister where she can stick her "requests", but still wants to attend, then I'd go with him out of solidarity.
But if he doesn't stick up for you, then he can either go by himself or not at all. If he chooses to go by himself (which I can actually understand, it's very hard to snub a sibling's wedding, even if they are being a cunt) then I wouldn't be happy about it, but I'd accept it.
However, if he chooses to go no contact with them over this then that is his choice. Your guilty over your mother is yours to own (or learn to give up on) - but he won't necessarily feel the same way, so don't hold him hostage with your guilt, let him find his own way and make his own choices.