Short answer: Yes
Long answer: If you do share it with your great grandpa, and you're using the same machine, then it's hard/impossible to detect, so you can probably do so without any harm. Keep in mind though that if found out, it will be dealt with. Especially if your great grandpa is a douche who can't behave ingame this presents a problem for you, as you are responsible for being the only person who uses it.