Nic,
In regards to what is allowed at Mum's house shouldn't matter. I believe that the girls should respect that isn't allowed at your house.
Totally understand and agree with you CBG, but in the heat of the moment, I can understand them slipping up!! I still slip up in front of my MIL who doesn't approve of swearing at all!!

I'm assuming that there is a lot of swearing going on at DSD1's school because I've seen some pretty colourful MSN / Facebook chats that she accidentally leaves on the screen when I am walking past...