I think this is more of a case where the idea was funneled into the player.
You're free to disagree though.
FE6 is the rare case of where the sequal comes before the prequel, but the evidence points to Ninian being Roy's mother(literally the plot), Lyn being Su's mother(Rath is her father, Rath doesn't exist in the feuniverse(:P) without her being with him, as well as fits her following in her mother's footsteps of marrying a plainsman). Lilina is the only odd one, but Farina is more than likely the candidate of choice when the game was being developed due to Lilina and Shanni originally being related(Ilia), which would rule out Sacae entirely(Lyn included), and Farina is only recruitable in Hector Mode, by Hector, while Florina exists in every instance of the game.
I try to keep my personal bias out of pairings (mainly because I don't really have one outside of serra x hector), so if this helps in figuring out why I made these decisions, then I'm happy.