We must not forget one fact, from which position Alexis acted, how she acted (I too was and am very irritated by her behavior). She was, and I still maintain, she is the “third wheel on the wagon.” It's just very hard to step out of the shadow of that kind of relationship that the MC and Erynn had and say, "Hey, I like you." Especially after the first kiss. How important this kiss is to her becomes more than clear later when she reacted very "allergically" to the MC daring not to remember this first kiss. But then he made up for it by even remembering the shirt she was wearing back then and that she still has.
You also can't deny that she does everything, and really EVERYTHING, for the relationship - not just in terms of physical love. The MC's influence on her is so extreme that the other Monroe family members notice. Layla told the MC that she had no relationship with Alexis before and now they talk about everything. Later, Adrianna assured the MC how much Alexis has changed and how good he is for her, which the MC is unaware of. And I suspect, with the last scene in EP 12, that Alexis' father also becomes aware of it.
Alexis' problem is that she is still not 100% sure about the matter, and the inner conflict - on the one hand, she doesn't forgive a rival next to her and on the other hand, for her the MC is "the jackpot"! Alexis hasn't invited any boy to her house yet, the MC is the first! What makes matters worse is that Erynn is still in the MC's orbit and now Iris, the MC EX, is her roommate. Erynn told the MC that Alexis isn't as strong as she pretends.
It is true that this relationship is still far from being called "normal", but it is developing.