Domestic Violence

Gather evidence and report him to the police.

That wasn't really what I was asking, but thanks anyway!

In an ideal world, I would report him, and the police would be arresting him today. However, I need to convince my mother-in-law to report him, because otherwise if the police knock at the door suddenly, and ask her, she will refute it out of fear. That also puts me in a difficult situation, because I would have made the claim.

I need to know what will happen if he is reported. I need to know the process so that I can explain this to my mother-in-law.

The only way I can collect any kind of evidence is to either witness an attack, somehow record it, or take a photographic record of her injuries over a period (which I think is ridiculous) He's only attacking her behind closed doors. We only see the end result of his attacks.

How about putting a nanny cam in her house so you will have video evidence of the attacks. Then simply turn him in yourself and hand over the evidence.
Personally, I'd be sticking my foot in his a$$ for even laying a hand on someone that old but, anyway a covert cam would be a good bet.

Believe me, I've had to dig deep to not break his nose. I don't want to ruin my chance at getting residency next year though.

I've been thinking of where I could put a camera. I can also put together my own setup with a Raspberry Pi and get one of those micro camera's, but I need to make sure I have somewhere I can put it where he's not going to notice it.

I'll bet and yes, I totally understand not wanting to mess your residency chances up or worse, end up in jail in a foreign Country.

I've seen everything from teddy bears cams to alarm clock cams to smoke alarm cams and everything in between. Plenty of good options out there.

Good luck, I hope that you get the evidence that you need.

Brother in law and mother in law is both from your country or is Japanese?

Both Japanese.

I find it strange that you are living with your mother-in-law and brother-in-law. May I know the reason? Is it because your wife wants to live with her mother? I can imagine your difficulties with this situation.
You are in an awful situation I am afraid, what more with the language problem. There isn't much you could do if the your mother-in-law doesn't want to report to the police over this matter.
My comment here may not be a good one but it would be my thoughts over this problem. I would move the mother-in-law with wife to live in another place far enough not to meet this horrible man. Don't get involve with the legal matter yourself because you are not familiar with the Japanese laws and culture.
I think the house belongs to him if the old lady dies, am I right? I don't know the inheritance law in Japan. Does the daughter(your wife) gets any share at all?
I have a strong feeling that Japanese women expect men to beat them  it is sort of old culture. The Japanese women see domestic violence differently from western people.
Lastly I am not Japanese so I can't figure how the women feel about violence against them whether from their husbands , lovers or sons.
Well, I guess the immediate problem is to get away from this bad situation and solve the problem later. At least you have peace of mind to figure out what to do.