I have been NC from my dad for approx. 5 years due to his abusive behaviour, mainly verbal & emotional. My siblings are more accepting of his behaviour (he is like this with them too but less so than he was with me - probably because I used to try to stand up for myself).
My SIL and DB have been married for approx. 20 years. They have 2 DC, 14 & 12. It is generally accepted in the family that DB is very much like my dad in that he is misogynistic and has a very bad temper. Since my epiphany with my dad and recognising that his behaviour is not normal and is in fact abusive, I now see more of it in the relationship between my SIL & DB.
She knows I think his behaviour towards her is unacceptable but says she loves him to bits and has always vowed never to get divorced from him as DB (& I) have divorced parents and she thinks that is the root of DB's "issues". Except that I can now see the youngest DC started to speak to her the same way and therefore history is repeating itself.
Through reading threads on MN I am coming to realise that she is probably in an unhappy relationship (probably because she may not know it yet) and is being emotionally, verbally and maybe physically abused by my DB.
SIL doesn't have much family and we're pretty much all she has, which is a sorry state of affairs as I have realised that most likely she's married to an abusive twat, all his family know it and yet we let it carry on. 
I have tried to speak to my mum but whilst she agrees that DB is probably exhibiting the same damaging behaviour towards SIL, she holds SIL responsible for not making DB seek help (despite knowing exactly what it was like being married to my dad.) 
She also said that one of DN told her that DB hits SIL but "that was a long time ago". So it just shows that my mum will not be a useful source of support to SIL.
So I'd like to know from some 'MN experts' how I can help SIL because I realise that it's not as simple as pointing out the wrong behaviour and expecting her to do something about it. Is there anything I can say other than just to let her know that I'm here for her & she can come and stay any time she likes and I'll stand up to DB for her?