The usual failure mode with the PS3 is that the graphics chip (and/or other chips) looses contact with some of it's pads on the pcb. They are all BGA (ball-grid-array) surface mount devices and the repeated thermal load breaks some solder connections. I have had limited luck following the instructions that one finds all over youtube and used a heat gun to reflow the connections. It's a lotto game. Sometimes it works, sometimes not. Sometimes the fix is good long term, mostly not.
Most likely your sister didn't do anything to break the player, they have a history of doing this. Search youtube.
Cheers,
James