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. I never understood when I was little but my Aunty asked us not to say "Oh My God!", "Oh My Goodness" was used instead. To this day I don't say Oh My God. Its the same with DP, being a tradie he'll say every word under the sun on the phone, but when it comes to talking to me - never ever says a 'swear word' to me...

Hate is a very big dislike of mine... I always say Hate is such a strong wrong, perhaps you don't like x at the moment.

My Step-Sisters children often 'gutter talk' - not much you can do if the parents allow it - they just know that I won't tolerate it and I suggest others words for them to use.

Lots of memories coming back re: this topic... "A swear/inappropriate word is used when you haven't got anything intelligent to say or use".

IMO or contribution to the topic