Blood Type?

My father is O + and my mother is AB . They had equal twins which is my sister and I, which came out O +. Everyone have been axiom that it's impossible that an AB mother could have an O + kid. We call up genetic counseling 'cause my mother had to step through them from the drugs she was on. We be the first babies to be born while the mother was on the blood thinner call Kumadent or something like that. They said the pills isn't an issue and my mother says she know for certain we weren't switched at birth. She have my sister natural while she have me censeran. So, how can this be?Blood Type?
yes, it is impossible for an AB+ woman to have children that are not any type A or B. and coumadin does not affect blood type at all. adjectives it does is make the blood clotting proteins not as alive, thus "thinning" the blood. blood types are found on the surface of red blood cells and are detemined by your genes. did you confirm the blood typing experiment results by having another testing for all of you (your sister, father and mother)? if you have confirmatory tests, you should look at other option and possibilities.
Is it possible that your mother's blood type is actually NOT AB? It wouldn't be the first time that blood exam was done incorrectly if it be.
the blood thinner is coumadin and it wouldn't be a problem. From what I understand you mostly end up next to Dad's blood type or a combo of the two. My mom is A+ Dad is O+ and I am O+. There is a long winded explanation at the website below. The website explains hidden A contained by the blood, etc.