The plot thickens Erin! You mentioned you are your sis's boss. I'll be that part of her resentment is that she doesn't have a health issue to deal with and is there when you CAN'T be and she feels that maybe you should switch positions? Just a thought, but working with family is very difficult.

I had a girl try taking a lot of time off with various excuses and when I talked to her about it she through my attendance at me. I very calmly explained that mine was due to my disease and that the owners were very aware of my condition, and that that was no excuse for her lack of attendance for all kinds of different (and I know made up) reasons and proceded to complete her written warning.

I didn't feel at all calm though, I wanted to scream at her to try to get through what I was going through and challenge her to make it to work for even 2 hours in my condition. She was the only one that was anything but supportive to me in my office.
I since had to fire her for other reasons (theft!), but it just goes to show how some people's minds work.

I'm sorry it's your sis you're having trouble with, though! That's really tough.
You've got the completely right idea though - take care of yourself, first! And blow off the bad attitudes you run into as quickly as you can.

Love to you - glad you're feeling better about it now!