Chachi

This isn't so much speculation, I don't believe, as I know it to be fact. However, I am also aware that this is generally accepted as impossible. My aunt works at a hospital and when brought into discussion, was told that it was impossible; my College Bio teacher told me the same thing when I asked her about it. My grandmother's blood type was AB; she'd undergone numerous cancer treatments and had been in the hospital often enough that I think we'd know her blood type pretty well. All five of her children (three males, two of whom are twins, and two girls, if it matters), however, have O blood type. I can't necessarily vouch for four of them, but I know my mother most certainly has O blood type, being prone to hospital visits herself. However, even one of her children having that blood type should be impossible. When asking my Bio teacher, she mentioned several other students that had asked her the same thing, but as the supposed father had AB blood type she assumed that the mother might not have been entirely honest. However, it's not like my grandmother could have faked a birth. So, I'm curious; clearly, if this is possible, it's one of those rare events that you won't read about in a quick genetics overview. This isn't a homework question, this is something my family has wondered about for years (having a negligible relationship with their mother, my mother, aunt, and uncles all joked that they rejected their mother from the womb). Any help? Some kind of explanation? EDIT: My grandfather's blood type is O. When my grandmother had to get a blood transfusion, none of her children could help her.