too strict?

Last night I went to a Taste of Home cooking show with some ladies from my Ward. We had a really great time and it was wonderful to go out with the girls. When I got home, I found a note written by Charlotte (who babysat) detailing how awful Jillian and Maggie had been to her...calling her names, insulting her, refusing to go to bed etc. This has been a common tale with these two and before, we've punished with their privileges (computer, Wii, tv) being taken away. This seems to no longer be effective because they keep doing it. So, this morning, we punished with something new. For the entire day (except for at school) both girls are not allowed to talk. They can talk only if we (the adults) talk to them. Casey made a litte tag to put on their shirts that says " I chose not to say anything nice, so now I can't say anything at all." When they get home from school, they will wear them the rest of the day. Needless to say, the girls were horrified. I know it sounds harsh but we don't know what else to do to get them to understand. Now, the next time we leave them with a babysitter, we can remind them of their day of silence and how horrible it was not to be able to talk. Perhaps they will think twice before being so cruel to their big sister.