Hum, just started the game anew and noticed something.
When fooling around with Lucy for the first time during the "drunk" sleepover event. Not calling "Iris" in the bedroom does skip the threesome, but doesn't change Lucy's behavior in the morning.
She sucked Iris' boyfriend's dick, but wouldn't have had interracted with Iris herself in such a way. So why would Lucy having a crush on Iris be relevant to "what we did last night"?
Out of context? Totally.
Spoiler alert
In context i have more doubts. Like the three of them hang out/get drunk together, Lucy and the MC fuck and sleep together. When Lucy wakes up she's like "No i don't regret what we did last night because i totally have a crush on your sister too."
So yeah hanging out with a crush is not something you'd regret doing i suppose. But fucking her brother who also happens to be her boyfriend behind her back while she's supposed to be drunk isn't exactly improving one's chances. I suppose it could, if it'd be brought to Iris' attention and would make the incestuous couple break up, but Lucy's personality doesn't feel that devious to me.