It will most likely remain a slow burn. It would be extremely weird suddenly to shift to sixth gear because something finally happened between them. There will probably be feelings of guilt, shame, embarrassment, or whatever else. Based on the mom's personality, I can't see anything with her moving quickly.
At one point, early on, it looked like Liane might be a LI. There's even a lust point you can get with her, and she has romance points defined as a variable. But there haven't been any points provided for her since the party, so I am assuming the dev changed his mind. At best, she ends up being another LI but the dev works on her path after he finishes with everything else in the game. I do have a set of saves created for her just in case, although I haven't played on that route for a long, long time.