Actually, its quite possible...
Its a case of multiple alleles (you can find this in any 1st year Biology textbook). It works pretty much the same as dominant/resessive genes, except there are a few more possibilites. You have I(A), I(B), and i. If you have type A blood, you could either be I(A)I(A) or I(A)i, same for type B. If you're type AB, then you have I(A)I(B), and if you're an O, you have ii
Now, in order for you to have aunts that are type O and type A blood, and for your mother to have type B blood, your grandmother must have had I(B)i, and your grandfather I(A)i. Now if you work out the punnet square, you get 4 different possibilities, I(A)I(B), I(A)i, I(B)i, or ii; type AB, A, B, or O.
So assuming your grandfather was I(A)i, then your family's blood types work out just fine (as any blood type is possible for your grandparents' children), and your aunt likely wasn't adopted. Hope this helps!