Not quite in the same vein, but the way my friend managed her situation might be of use. When her son's first daughter (and first GC) was born, she was kept at arm's length, the DiL getting help/input etc from her Mum, which is generally the most usual scenario whether there are rifts or not. The son would randomly call her to say, for example 'you can come down for half an hour now if you want' or 'you can come down now and bath her if you want'. She wasn't allowed to just pop in. She wasn't allowed to buy odd things for the baby without approval (anything she bought which the DiL didn't like was given back) She was working shifts at the time, and on her own, and whether she'd just come off a night shift, or put her car away for the night - off she'd go. I used to urge her to tell her son how upset and left out she felt, because these visits were irregular with sometimes long gaps between each, but she wouldn't, saying she didn't want to cause any upset and was grateful for what she got.
Well, as so often happens, the novelty wore off eventually, her son began visiting with the baby, she was invited over there more frequently, and then the babysitting started, and before you could blink an eye, they were quite happy for her to have the DGD over the weekend!!
At the moment, you are the baddie. Maybe the girl is being so controlling and 'nasty' because she's scared of losing your son if she allows him to be 'influenced' by you, frightened of losing the baby in some way, especially following previous loss? Does sound, though, as though she could do with some counselling.
As others have said, and indeed as you are trying to do, just keep that door open, be patient, support your son and advise him as best you can and maybe the girl will grow to trust and like you eventually. You never know! It's a difficult and sad situation, but take heart and keep posting - sometimes a solution or course of action materialises just by talking/writing about it. Good Luck to you all.
