^ Yeah, Morgan is very clearly Robin's offspring, but it's mentioned in passing that they come from a different timeline. Probably because Robin can also marry the second-generation units, which would open up the question of how they got back in time when their parent already went back in time before meeting Robin.
^^ I'm not sure if any of their potential siblings remember growing up together because I only ever played FE:A with a friend during sleepovers (gosh, it's been 12 years already?) but the wiki tells me Morgan only has supports with second-generation units as either parents or siblings.
Also, just plugging the Gay Awakening Mod here for all who don't want to speak in mere hypotheticals about WLW (and MLM) reproduction.