I need serious mature advice on how to handle this situation...?

First of all I'm very sorry to hear that. And as for my advice, first talk to her doctor on any alternative medication and if there isn't then come up with something that could subsitutie them like a favorite activity she used to have or do with you. Maybe you should suggest she moves closer to you? Even though she might refuse, as long as you and everybody you live with has not done anything to upset her drastically, it's best for her over time to be near family or just to know you guys are there. Also, with her husband, I think he should give your sister space while you and your family figure a subsitute, past time, or alternative to the medication. If he loves her like you say he does, he'll be patient and once she is better or starting to become herself again they should sit down and discuss somethings. Like how he delt with this and why she was reluctant in anything she has done (if any reluctants just trying to give some examples) I hope everything works out okay. Good luck and hope this helped.