I think in general, 14 year old would rather live anywhere than with their Mum. If it wasn't his sister, it could well be a grandparent, uncle, father etc.
The comments your son is making, are awful, but most teenagers say horrible things. Just look at the threads on teenagers and you can see they all say nasty things!
If your daughter wasn't involved in all of this, it would be hard, but not uncommon. It is because your daughter is involved which makes it so much harder. The fear of your son turning away from you, like your daughter makes this a nightmare.
As for your daughter, she is angry. There may be other things happening in her life and she is just blaming you for all of it.
When families truly become estranged it is due to severe abuse. Keep the door open for your daughter, and i do believe she will return.
When she has children, she will realise how hard it was for you and value you then. The door will be open for her then. But I do think it will be sorted out sooner than that.
Take care and keep on posting messages for ongoing support x
You know you done your best in difficult circumstances.