I know that when I find the people in my life with behaviors similar to AH the most frustrating it's usually bc something about my AH is feeling especially out of control.
For ex. When I let AH's brother really, really get to me last week it wasn't bc he was being any different than he's always been-- it was bc I was a lot more frustrated and fed up with AHs antics. It's "unsafe" to be uspet with AH so I focussed my frustration on bil. I also was/am feeling stressed beyond belief about my job being cut, my financial state etc... So, add in bad behavior by bil and that gives me a different focus.
Not suggeting that you're distracting yourself bc your sister sure sounds infuriating but just something to think about:
Is there something about the A in your life that is feeling more out of control than usual?
Is there something going on with you that you're having a hard time dealing with?
These are things I've been trying to ask myself when I find myself wanting to lose it on a sick family member (ie: last week I lost it on my mil. I am glad I did, but I prob wouldn't have if AH hadn't been pushing my buttons in a particularly huge way...) There seems to me to always be a connection between my reactions and what's going on around me and ultimately I need to find a better way to focus just on me.