
They never call themselves "friend with benefits" and it's just an assumption on one of the guest. To be fair, I wouldn't use this term to describe their situationship as they hardly have any "benefits" moment shown as of now. They are more like in a relationship without label or in the "flirting era" before dating.
Is that cheating ? Maybe, maybe not. But it's 100% a breach of trust and a betray from his own brother as it's pretty obvious that she have real feeling for him & he didn't say anything to her before "playing" with her brother. Dare to tell you wouldn't feel betrayed if it happened to you.
Anyway, it's just a really bad situation with horrendous timing that will hurt peoples in the process as I wouldn't say they are bad for falling in love with each other but they handle thing pretty badly.
Ah yes normalizing cheating...