This is a very complicated situation. I'm not sure if this is the right forum, but here goes.
My MIL and PIL jointly care for their adult disabled son (in wheelchair). I have known for some time that PIL has a temper, and that he and the son often have fierce arguments. My husband doesn't seem to want to get involved. Recently MIL recently told me that occasionally PIL behaves terribly to their son (she gave me some examples and it appalled me - calling him a cunt, punching the pillow only a few inches from his face) and on a few occasions PIL has hit him.
I think she would leave him if she could but she has bad health, is sometimes in hospital for several days at a time, so there is only PIL there to get the son out of bed, dress, clean him etc. When they're getting on, they are quite civil to each other, he takes him to the pub and on day trips. I think it's when he's tired or drunk he loses his temper and takes it out on their son.
I have always thought that if I ever learned of someone committing domestic violence I would try to help and step in, but what can I do here? They live at the other end of the country from us so we can't help with caring for the son. We don't visit very often (partly because of the bad atmosphere) and I feel I can barely look PIL in the face now.
To complicate matters MIL also said he is "getting better" because she's started standing up to him. So perhaps I should leave things alone. But I do know it was happening as recently as last year.