ok so when they found Morgan it would have been like a few weeks after the accidental affair with Walheart and that is how they found out she would be pregnant with Morgan bc white hair??? calling Robin Mom?? CALLING WALHEART DAD????! UH YEAH YOU GOT PREGS GIRL, and it explains why Lucina would say she never had a full sibling, they always just assumed she meant she seen one of the other kids as one UNTIL THIS HAPPENED
Robin would just curl up and be like 'oh fuck fuck fuck fuck me why-- this is why I dont get drunk when I have fought with someone shit'
and just it takes a lot of talking to Morgan for her to come around to the fact that it did happen and he would be born in a number of months, she dose eventually and relizes she got the best son ever
and at first Chrom is just oh gods my wife really did do the thing with walheart and Walheart was braging about it but when Robin found out she would take the blunt end of Walherts axe and hit them both over the head
and Little Luci never cared she was just excited to be a big sister, but hey in the end it helped make Ylisse and Valm closer in the long run 8D