Simple/basic system to play with my little sister

Hello everyone. Im looking for a simple system that can be played by something like 3-5 people, which is simple enough to be played by an 11 year old. My sister is in her Harry Potter phase so magic and fantasy wizard setting is really helpful. I need it to be simple enough where I can teach it to my family in a night, and we can get right into whatever story I end up DMing (for the first time) for them. I really want to spend some time with my sister over the summer engaging her imagination.

Suggestions please :D

Edit: you people are wonderful, I'll check all your suggestions out for sure!