OP, my exBIL will tell you that SIL was abusive, toxic, manipulative. He will point to her losing her temper, "making him homeless" etc. But he'll never mention that when she lost her temper it was because she asked him time and time again to step up, to do childcare, to help in the house. Because he said he'd look for a job... but didn't look for a job. When she "made him" homeless, it was after she allowed him to live with her, 100% rent and bill free for a year while he was, supposedly, looking for a job until eventually she kicked him out.
He also went through a phase of telling anyone who would listen how violent she was to him. But he doesn't tell you that this so-called "violence" was when he forced her bedroom door open, grabbed her phone, went through it to read her private text messages and then refused to give her phone back while she was physically doing everything she could to get it back.
He will admit to forcing doors open, and locking her in rooms.... but he genuinely, hand on heart, does not see why this is a problem. He once told DH that she "deserved it" and perhaps she'll have learnt a lesson about going out with other men (Not that it would ever have been okay, but for the record a) they'd been broken up for over a year already and b) she wasn't going out with other men). Listening to him, or reading his messages he sounds so SANE and genuine, it's jarring when you stop to think about what he's actually saying.
My point is that while I am not someone who believes that where there's smoke there's fire, there is no doubt whatsoever that the police do not come to a third party and warn them if there is any doubt whatsoever about his behaviour.
Also, with all due respect, this is man who has managed to get two women pregnant in quick succession, neither relationship has been very good... I think the reality is that he's NOT a good one.