All good points, yeah.
Layla and Alexis were not close when they were young, to explain why MC never saw Layla before, but they grew close during MC's absence. They are already close before MC comes back. That's why Layla can notice the changes in her sister's behavior. Everyone notices Alexis is happier, especially Erynn. Her father too. It's just that the scene in episode 11 makes it even more obvious to him that her current boyfriend is a good thing even for him
Alexis' other problem to me is that she has incredibly high standards for her boyfriend. MC has to be close to perfect, and make almost no mistake towards her. Prince charming, essentially. Even if she is easy going in the relationship and forgives him a lot of small mistakes (mainly because she just made huge mistakes herself, forcing her to be more forgiving). She has her parents' perfect relationship as an example everyday. As the relationship grows, it might become harder to maintain it.