dealing with someone mentally ill?

I have this soon to be brother in law, he's schizophrenic and bipolar. 37 years old which is only a year older than my self. This guys is a thorn in the family, his parents are in their late 65+ years, the other brother and sister don't know what to do with him short of taking him outside for a beating three or four times a week. He spends his parent's money behind their back, even though he doesn't pay rent and gets a disability check for 800$ a month. Most times he's harmless doing really odd stuff like taking the bricks out of the foundation of the house he lives in just because its a different shade of red, then there times he just gets in the way, and I have to beat him down my self just to get him back in his right mind. He's on medications to suppress his dopamine, but he drinks caffine drinks to the tune of 72 cans every two days. Which if you didn't know, increases the reward neurotransmitter to just enables him further to do what he does.

He needs to go to a sanitarium, but I don't know how to tell the family about how to do that against their ill brother's will. Does anyone have experience in this and know what to do.