When i read your texts here, it sounds to me that your in laws have no idea what they are facing. Assuming what the brother does have SZ, his parents' denial is making things so much worse. They need to at least find some literature on the subject and get some-what educated on this difficult issue.
If one is suffering from phsycosis, large family gathering, for example, are always a triger. I have never seen psychotic pations getting better during or after such gatherings, only worse! (I am not a doctor, i just have 3 close family members suffering from sz, including my mom, brother and daughter)
Furthermore, where your in-laws can only GUES that this or that would be supposedly good for him, but for each of such "guesses" there are probably plenty educated studies and answers in appropriate literature and they need to educate themselves rather than continue taking guesses.
However, if they have made a choice to play doctors and keep guessing what's best for him and forcing their unprofessional opinions on to you, then all you can do is just stand your ground.