It's seemingly a very simple game. You play through, do puzzles, defeat bad guys, blah blah blah... But then, after you've completed the game, you go back through and unlock a TON of extra characters and spells and whatnot. It's a great game because "Free Play Mode" is completely different from "Story Mode", and you won't even qualify to unlock all of the achievements until you've played though on Free Play.
My only complaint about the Lego games is that Free Play mode can be kind of complicated. You definitely need a walk through up on your computer while you're "treasure hunting", or there's really no way you'll find everything you need to find.
Either way, the Lego games are fun for everyone. Your kid sister or your grandfather are both bound to enjoy it. In addition to Lego Batman and Lego Harry Potter there is also Lego Star Wars, Lego Indiana Jones and Lego Rock Band.