Darling OP, sell the flat, there are a million beautiful places in France for you to go. You don't have to go through all this, really you don't.
A holiday home is supposed to be somewhere you to to relax, not somewhere you feel stressed and sick with worry you will bump into someone....
Once you get this situation back on even keel, you can try to manage your sister.
However, if she starts to threaten your family/life etc, then cut all ties. You can alway re-establish if she learns to behave, if she doesn't then the most important people in the world, you family are protected.
She has alienated almost every living soul she's ever met.... are you not seeing the wood for the trees? They have all taken the decision to bail on her unaccepatble behaviour, why do you have to suffer?
Your mum had an inkling of your sister's behaviour, and this is why she said to keep in touch, because she knew that if she didn't say anything, with behaviour like that, only a fool would stay in contact with her.
Your mum is gone, you have done your best, but your family is being threatened because of the insanity of your sister. You have to stand up for your DH, your DC, especially when it goes past a certain point.
Sell the flat, give it one more go at standing up for yourself and not taking her disgusting behaviour, warn her that you will do something about it unless she changes, and then follow through when she doesn't.
You tried to do what you have been asked, but it's not viable, and it's beyond your control.
2010 is the year you reclaim your peace....