cheating girlfriend what a side dude to do

Is it okay to sleep with a cheating girlfriend of another guy when you know it? When they are in an LTR?

So basically met this girl at my sisters birthday rooftop party . Kicked it off smoov, like I'm talking about she was giving more attention to me than to her boi, we were laughing and having great conversation. Anyway the night ended , My sister gets a text from her saying "low key your bro...