I think concept-wise she's interesting, for sure! Execution-wise, she wasn't quite what I was expecting. I thought having a sister would serve so both her and the MC could bond with Eren and Mikasa. Yet Eren is pretty much non-present, and Mikasa's route doesn't really need Elsa to work. If anything, it seems like Mikasa is who ends up being used as a way to kickstart things between the MC and Elsa.
If you tell me you have a game where you can romance Mikasa, my first thought is, "damn, she's going to be a tough nut to crack." If I then read in the description that the MC has a sister, I go, "oh, that's genius, because the parallel sibling dynamic is a good excuse to make these original characters get close to the mains of the source material." But when I see that the MC got close to Mikasa pretty easily (not that I dislike her route, I do like it), I kinda question the need for Elsa's character to exist in the first place. Unless it was to still have some incest for the MC to indulge in/be conflicted by, with you not playing as Eren. I didn't really like her route, and was hoping for a choice in which I could reject her to be honest (kinda like the one you get with the instructor, where you get a warning that the choice will affect the storyline of the character).
That said, of course this is up to the current available content. It seems like you were going to have Connie going after her during a couple of scenes, I think something like that which makes the more protective insticts of the MC kick in would still be interesting to see. Something involving other male characters. And of course, you have the thing about your mother being "missing", that makes me wonder what you have planned plot-wise to end the game. Personally I'd to see this cast of characters be put on the Battle of Trost when all hell breaks loose and see how things play out, specially if you were to get some choices during it. When you bring death into the mix, things can get very interesting.
So my tl;dr opinion on Elsa is that I'm not really vibing with her at present, but I can see her potentially being used for interesting stuff in the future.