Tiny radio controlled aircraft are all the rage these days, but they're one of those toys that can be tons of fun for the user, and pretty annoying for anyone else near you. The temptation to try and buzz people by flying near their heads is just too strong for most of us to resist.
The MX-1 from Silverlit is by far the smallest R/C heli we've seen, which means it shouldn't cause too much damage when you crash it into things, although I would still recommend avoiding your sister's head. Measuring a mere 4.5 inches long, the MX-1 actually fits into a compartment in its own controller/charger, where a full 15 minute charge will give you about 5-7 minutes of flying time. The controller is a fairly basic two channel model, which makes it easier to operate, although ultimately not as maneuverable as a more advanced four or six channel heli.
The MX-1 is available now for about $50.