You are doing a good thing OTWO ,but it zaps your energy .
My advice to you is Be straight with your sister about her husband visiting .She is lying because she doesn't want you to be mad ,so have a calm discussion of why it is a bad idea to allow her husband too much leaway . Remind her of why she is in the home in the first place . Sickness may indeed be effecting her mind ,so you may need to have this conversation over ,and over again . Tell her your fears be honest with her too .
Sometimes you just have to realize you can only do so much, and then let it go . It sucks that you end up being the person that will have to pick up the peices if she makes a bad decision . It also stinks that you will be labeled the bad guy ,but what matters is that you know the truth and that you did what needs to be done .
I am in a similar circumstance with my sister . I have to make unpopular choices ,and get labelled as the mean one too. It is hurtful when you do so much and then someone can waltz in for two seconds and throw a wrench into all you have done . My sister will lie to me too, because she hates it when I get mad .
I hope you can get a break and just not think about it for a day .