I am also not understanding Rhea liking Ranbir. She wasn't in love with Ranbir some times ago and now suddently wants to ruin someones life for him. I just feel she to doesn't like Ranbir, its just infatuation or her ego is hurt so she wants to ruin Prachi.
The first time rhea ever showed interest in ranbir was when she got jealous seeing ranbir talking to prachi in college right before rhea told principal to suspend prachi from college. Till then she never even looked at or talked about ranbir. Ranbir is the same in that he also never thought about or talked about rhea until aryan gave him silly suggestion to date rhea just because she's the most popular girl.