Little sister,
When a person who is used to being responsible for their own actions and decisions is faced with uncertainty and lack of control over even the most ordinary daily functions and decisions the result is great frustration and anger. It is comprable to a king being forced to be a stablehand who just cleans the stalls and feeds the horses. As a police officer your brother dealt with life threatening choices as a matter of course but he had framework for making those tough decisions on the fly. Now he has lost that and is at the mercy of the unknown and his mastery of his own life has been lost. Also being inside when you are used to being on the go is depressing whatever the reason.

You have received some excellent advice here. It is very important that both your mother and brother have some pampering and distraction. If the TV is always going - unplug it and get some silly videos to watch, the stupider the better. Elmer Fudd, Casper the Ghost, Andy Hardy, Roy Rogers, Monte Python, I Love Lucy. Get out old family photos and try to figure out who everyone is. Start working on a family history. Get some games to play. Try to play music with out words. It doesn't matter if it is classical, jazz, rock whatever he likes. Take your mother out for a ride to nowhere, perhaps to a nice park, water area or pretty neighborhood. Give your brother some problems to solve for you, ask his opinion about something in your life. Do whatever you can to bring variety and joy into his life.
Treat yourself to something like a really cushy robe or some extravagantly expensive candy. Find something to read, watch or listen to that will take you into a different world for at least an hour.