@Rrrob
FIL said DH and I were genetically incompatible after dd was born (unexpectedly) seriously ill. I will never forgive him.
Oh love you are still hurting yourself and almost certainly with no reason. Your words " I will never forgive him" mean such pain in your heart it comes off the page.
Would you mind if I tried to suggest an alternative interpretation? When he spoke, your brain was not in an academic lecture hall, calmly, rationally and dispasionately assessing bits of information, selecting various interpretations or conclusions to be drawn.
At that moment, when you were raw with sensitivity after the shock and worry of the birth, of course your brain was very liable to be hyper-protective, hyper-defensive, shoot first and ask questions later.
The man probably had a half-grasp of scientific knowledge, which wasn't intended as an insult of some kind. He could easily have heard some scrap of information about genetics. He might have vaguely comprehended and that in a large range of inherited conditions, either parent would not pass on the gene, provided they did not marry someone who was also, unknowingly, a carrier of the same, recescent, gene.
What he possibly had not grasped is that just because a baby is born ill, it does not automatically suggest the illness is genetic. And in any case, even for some genetic conditions, there is huge complexity about the outcome of the parental pairing matches.
Isn't it far more likely the FIL was, yes, clumsy, hastily-spoken, and half-informed, but not ill-intentioned at all? His brain, too, was in its own however dim way, only half working that day. He too must have been horribly shocked and frightened, when he heard his grand-daughter had turned up in the world unexpectedly, and seriously ill.
Everyone of us is at times tactless, ill-informed , the more so when our brains are fogged up with emotion. If you re-interpret FIL's words as only that, clumsiness, not as deliberately intended malice, it will remove the burden and pain you still feel.