Mare in foal?

I am looking at a 6 year old registered Paint horse mare, in foal to her HALF BROTHER!!! She is very sweet with one blue eye, her previous owners were neglect full and left her and her brother in the same area and she got pregers. She is an AMAZING $200.00. But that hole half brother thing unnerves me, can the foal still be registered and will it be NORMAL as in physically and mentally.