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.