Sister was diagnosed by her doctor who recommended that she have live in because she was unable to take care of herself, i.e., cooking, bathing, etc. daily care.
You would have to provide MORE than half of their support. So if sis gets 800 a month in Social Security, you need to prove you spend MORE than 9600 a year supporting her. Now if you live with her, that is going to be hard to prove, since she is providing you with shelter.