As many others have tried to explain to you, the biological components of a human being are different from human acts of will. The fact that some of Mary's ancestors may or may not have practiced polygamy could not possibly affect the single egg she may possibly have released to engender the Messiah; or it is also possible the entire embryo was implanted by the Holy Spirit. Either way, the actions of forebearers are not transmitted by genetics.
Polygamy is an act of will that in this thread is of concern because it involves underage children and incest. Polygamy is a behavior, not a genetic trait that can be transferred from generation to generation biologically.
You question has just given me more insight and I will post it later!