Having a descendant won't change a thing.
Having magdalene's DNA would only prove that the Grail is a descendant of magdalene.
(only in the unlikely event that you could get a good sample, given the fact that normally the time it takes for half of it to decay is about 521 years)
Of course nobody ever prohibited her having offspring
(or brothers or cousins at this point, because, you know, it gets tricky when you need to find connections of two people living 2000 years apart).
Anyway the church already denies her intimate relationships with jesus.
Whoever used to think that their demi-god was screwing a lady will probably have toyed with his human nature already.
Whoever never bought that, will keep on not buying it
So, in the end, having a living descendant won't make any *beep* difference