Replying to Family dilema help

so for my 18th my half brother invited me and my sister for a 3 day trip to paris with his family (live in England not such as big deal as from the states) however i dont really want to spend 3 days with his family as not such a fan of wife or kids, my sister in the same boat. he wants to go to museums and such (nothing which appeals to me) however me and my brother are pretty close and dont want to ruin our relationship because of this. any suggestions