I know all we can do is "be there for her" and she's free to make her own decisions.
But they have a dd together (5) and sil has recently broken up with her (from what everyone can tell) lovely ex fiancee. She and her exh are still married. My own personal concern is nieces wellbeing, but sil is a great mum so hopefully if things "turn" again she will have the strength to leave him. Again.
She hasn't officially told anyone anything but has included his name in her christmas cards and he is now always at her home. We literally never saw or heard about him from when they separated 3 years ago to about 3 months ago when she and fiancee split.
I know he took niece as much as he could (works away).