I think she over reacted. If she going to mad at some one, it should be him.
His wife is so silly. What is her problem? Is she that insecure? Were there other people living around or present at the house for her to take it so offensive?
If it was my husband, as long as it was at his sister's house and not at some other woman's house... I would have cared less.
My advice, let things cool off for a while like couple weeks, then call your brother up like nothing happened and asked if he would finish the roof. AFTER he finishes the roof then you can bring it up with the stupid wife to clear the air if you still care.
Actually she called and apologized to me already. She was more upset at him than me. She came up in a very conservative home and is not use to such openness. I did tell him to respect her wishes. One day I am going to take her out and get her drunk.
I had similar situation a few years ago. My brother and I used to be nudist. He still is, I went and got fat so I stopped going. Anyway,he paints houses part time and told me he would paint mine. Me and his wife were on the patio talking when he finished and asked me if he could take a dip. I told him sure . He went inside and came back out totally nude and jumped in the pool. His wife freaked and started apologizing to me. I told her to relax,it's only me. When he got out the pool she held up a towel and put it around him. I just laughed it off.