On topic, i'll admit the "niisan" in the trailer made me cringe a bit, although "imouto" type characters existed in Fire Emblem from the start (look at Minerva's story in Shadow Dragon and her little sister, up through Mist in Path of Radiance).
The ability for total intermarriage in Awakening didn't hurt the women, though, it hurt the men, who, aside from Chrom, became utterly interchangeable in that it only mattered that you got a specific woman married to get the child, and this then created a stronger character arc for the women because they could interact with the next generation freely, whereas the men became more stilted outside of their own character quirks.
Awakening definitely felt less well put together in the character department because of that, comparing to Fire Emblem 9 & 10 where many characters felt like they were really a part of the story and not just hangers-on.
14's plot so far looks intriguing to me, however. You have the player character, but they choose one side of the story and stick with it, meaning each side should be more cogent once the branch point occurs. I hope that the diverging paths intersect a lot, though, similar to Radiant Dawn where you had chapters where the Dawn Brigade, Crimean Army, and Greil Mercenaries ran into each other. Hopefully this mechanic will have chapters where, on the Yoshida side, you're fighting some folks from the Nohr, and on the Nohr side, you fight that same group from the Yoshida, with the only difference being the perspective (and obviously some gameplay tweaks, because it's hard to have a Fire Emblem game function as a match of equals, as the streetpass functions in Awakening demonstrated)