Let's forget about likely couples and examine likely siblings for a minute. Out of the all the future children, who is most likely to be Morgan's brother or sister?
Let's assume that both generic and unique supports happen for Morgan. Also, disregard official hair colors. That way Kjelle and Noire don't get an unfair advantage.
Ironically, I kinda have to nominate Noire. Not only because she and Morgan play off of each other really well, but also they have similar talents. They both have martial and magic skills, plus they share this odd talent for making crafts. Noire makes a series of talismans for herself, while Morgan made a entire box of masks during her support with Gerome.