Here's a list of things that exBIL has threatened SIL with, off the top of my head, that, at various times, have made her very nervous even though actually, not one is true:
1 that he will go to court and get custody of their dc (when they were TOGETHER he was barely present and since they broke up he sees the DC about once a month, at best, so how he thought this was a strong argument I don't know. except... she believed him).
2 He will tell the police/social services/court that she's an unfit mother, parties too hard and uses hard drugs.... she was partying about once a month, usually WITH him (pre separation) and while frankly, the mild drug use was, in my opinion, completely unnecessary and inappropriate, it WAS mild, DC were well cared for AND he was using at the same time. Sadly, a typical case of middle class parents blowing off steam on a weekend at great expense.
3 He will tell everyone (and/or police) about the violence she inflicts on him. He has recordings of argument x or y. Bizarrely, arguments X and Y were when HE was violent to her so that one made no sense and she didn't actually buy into it. On another occasion, after he broke down a door and took her private things, and she was grabbing and hitting at him to get them back she DID worry he'd go to the police but we reassured her that the broken door was pretty clear evidence of self defense. As was his LACK OF ANY WOUNDS. Particularly considering she had minor bruises at various times as a result of his behaviour.
4 Threatened to contact immigration on her/extended family because they're not English. They are, however, in this country 100% legally, pay taxes etc.
They threaten and bluster and they don't back it up.
There's a woman on here whose ex is sending her threatening solicitor's letters about "debts" she supposedly owes them from when they were together such as petrol for trips they took! Mindblowing.