Blood type question? genotype?

You need to figure out of they're zygous dominant, recessive or heterozygous for the same trait. So your dad is: Ai, your mom must be Bi (no pun intended please!) because your sis is an O. If either parents didn't have the recessive trait for O [which is (i)] your sis is probably adopted. So it does work out that theres an AB, O and a B.