Oh, come on! You really expect me to come up with such an OBVIOUS solution like THAT?!! I'm not THAT senile YET!
You know, it's just crazy enough that it MIGHT work! Dammit, I'm going to TRY it!
Uh, there's TWO sisters... So, "Sis 1" and "Sis 2"? (are their NAMES?)
Seriously, I don't usually try that because I start thinking that other people use the relationship for their NAMES. Dad may call his wife 'Mother", but the co-worker ISN'T, and her teacher isn't normally going to call your sister "Sis".