Just an idea, IF Mama Susan (quoting ItsaGundam LoL) allows:
When someone does a sexual thing to the protagonist, Yeah the one with the POV interactions, and he is not aware of it, It didn't happen, and well, technically speaking it is rape... My solution would be making him aware of what is going on, but make it so he keeps his eyes closed. Why? Cause he would not risk comfronting any of them, cause last time he did, he kind of lost one of his "roommates" and took him a lot of pain to reach the current "healthy" relationship. This if he considers that is the older one doing it.
If he considers that is the youger one, then he would want to confront her and having to expose the secret encounters or scare her since she is not stable, or making her/them ashamed and lose them cause maybe he does not want to crush the foundation that "he built" with the bathroom massages and all that. This matches with his current father/love interest/desire conflicts.
If you make the girls whisper, considering that he is around 40 tiinitus is a bitch, it would be more than plausible that he is not able to know who is speaking, with the storm and rain added outside the noise threshold would be high enough to mask the whispers, specially considering that the 2 girls are really close to each other.
There will always be holes but at this point any person who has been reading the story knows the MC, so there is no need to make him asleep (which in my opinion atacks suspension of disbelief) He can be "awake" without affecting the plot, if you are trying to give something to the player without scalating things too early. He woulndt make a big deal of it in this case is just added baggage, and still fits the logic of what is coming next, since she becomes more open.
But if what you are aiming for in that scene is just a lesbian scene with a dick as an excuse and just want to focus on the older one's desires, well, one idea would be make the little one play with the older one as they are, and instead of a HJ, she could just finger her or something and at some point the older one could bend over and rest her head close the dick, and maybe give into temptation and have a taste as part of the climax process. That could work pretty well if you want to tease, and works with the next encounter dialogue. A soft mouth in your dick will not awake you specially if its not a sensitve area, and sleeping with 2 people, having a body part on top of you would not awake you either since its kind of expected in a crowded bed.
Wow my imagination run wild here! The End.