MIL issues

When we found out I was pregnant with our second baby. We wasn't going to tell anyone at first, except our parents. Well my MIL told my husbands brother and his wife, his grandparents (main one we wasn't telling yet). When I found out I got extremely pissed. There isn't any telling who else she has told. Is it wrong of me not to tell her know the gender of the baby when we find out and make her wait til we announce it?