Unless you have some other backup arrangement for someone to watch your child on Mondays, I doubt there is much you can say or do that will have any effect other than causing resentment. Does your sister get paid to watch your son? If so, that may be the only leverage you've got. If not, and if you don't want your son around the brother, then you probably need to find another sitter. But to your question -- are you over-reacting? -- I don't think so. I wouldn't want my kid around him either.