Hypothetical Question?

Okay, guys, I have a quick hypothetical question. Let's say there's this girl, we'll call her Ali. Now, Ali has this brother named Keith, and Keith has a really bitchy girlfriend named Madison. Now, one morning Ali woke up and saw Keith making out on the couch with Madison. Now, besides being gross, Keith was supposed to be at work, but he sent his and Ali's other brother, Marcus, in his place. Ali knew her mother would be very upset. Now, hypothetically, would it be wrong for Ali to call her mother and tell her what's going on? Hypothetically of course. Does it make it wrong if Ali's just doing it for revenge for when Keith turned her in for coming in 5 MINUTES after curfew? What about if Ali's 17 and Keith's 19? Thanks for the help.