Would it be okay to take lessons on playing the piano to run a scam against my girlfriend's brother?
What kind of scam are you planning on the piano against your gf's brother?
My girlfriend told me her brother is looking for a piano teacher, he is super talented, but shy. If I take lessons with him, I could talk him out of money
First of all, that sounds like a ton of work just to scam a bit of money from him. Secondly, if you've got that much leisure time, spend it productively instead.
So? It's fun.
You should find another activity to find your fun in. If you're willing to put this much effort to squeeze out a little money and joy, then think of how much more you could accomplish doing something actually difficult and rewarding.