If Sarah can't wear clothes, how come she wears a big overcoat?

Well, partly artistic license. But also, Sarah has found that leather isn't quite as bad as other materials and metal doesn't cause her any problems at all. Her coat is leather and you'll note it's big, baggy, flows around her and she leaves it unbuttoned. I'm sure it kills her shoulders and arms, but the rest of it isn't really in any long term contact with her body allowing her to wear it. You'll note though that she seems to ditch it at the first opportunity.