Limerick,
As someone who is frequently in the position you find yourself in, I understand completely. I too have to pull out all of my detachment tools in order to be around certain individuals in my family -- and get very clear on my boundaries. You can't control your sister nor her husband's choices & behavior, you can only be there for your aunt....as Ago suggests, I think your best bet is to go through several possible scenarios of "what might happen" and have an escape plan mapped out for each. Your own car and place to stay will help a lot.
But mostly, just try to get through this with maximum serenity. It won't be easy but it is just a short time, and you are on a mission: be there for your aunt, ignore your sister's situation as much as you can, as there is nothing you can to do change it at this point.
Hugs to you to get through this
GL