The worst thing you could do is to cheat on yr girlfriend with anyone- let alone her own mom. How would you feel if yr girlfriend cheated on you with yr dad? Think and try to be a bit more unethical. You're not too young to know right from wrong.
If yr unhappy with your girlfriend, then by all means break up with her. But you need to stop this childish fantasizing about her mom. Even if she wasn't yr girlfriend's mom with her first loyalty to her own daughter(not some stranger) this woman is old enough to be YOUR mom and could get herself into some serious trouble if she broke the law by sleeping with a minor.
Grow up and leave this family alone. You may not care about your girlfriend anymore- but after 4 long yrs together, you should have more loyalty than to want to destroy her family life just because you fancy her mom.