I don�t think you are being unreasonable as such, but I do think you should be taking issue with your boyfriend and not his sister. If she doesn�t know that you are getting engaged, then is it quite likely that she doesn�t even know that you will be moving in?
I would let this one time slide after all a new paint job isn't all that bad � you can always redecorate to your own taste, once you move in and set up home together then she should not interfere and maybe your boyfriend � or fianc� as he will be by then � might have grown a spine.