[Shadow5YA]

Originally Posted by GDB

Spoiler for Ending:

In Lucina's future, Grima doesn't awaken until Lucina is like 14. That's plenty of time to have a kid, especially since Morgan appears at least a couple of years younger than the likes of Lucina and Owain, and there's the chance that Morgan was first transported to Fire Emblem 7 before then being transported to Awakening's time.

Main thing is, Morgan's siblings (at least Lucina and, especially, Owain) know who Morgan is. Owain especially is very protective of his little sister, and female Morgan's personality is actually quite similar to Lissa's.

But Lucina's sibing supports are all the same. A generic support conversation doesn't really help Morgan's case when she says the same thing with any other character as her sibling.

Morgan's sibling supports are generic as well, which also doesn't really help his/her case because he/she could just as easily have some unnamed character from another world as his sibling.