I worked on the sibling, and ignored Devon McFuckface pretty much, when he said I should share the mom I told him to fuck off. I'm not sure sleeping with her is a trigger, because I progressed with her even when I slept together with the sister. Btw I re-checked my save, Lust is at 41, Relationship is at 60, and I already have 1 Pregnancy with her. Try the beach, the pool, the work, the breakfeast, etc, except for the peek when she is masturbating, since that puts lust down by 1. Also check what gives lust and relationship because after a certain phase, not everything increases it. Maybe you missed out on something like the beach, and have to do that twice. I remember that at one time you had to shower with her as well. Idk how far you progressed at work, but you should also get promoted at least once I think, I had that twice, it helps you gather money faster anyway and MC mentions it to mom as well. Physique limit also gets bigger after some phases, I have mine at 165 now, maybe that should be increased as well if you can.