Problems with gf sister?

Ok, so my gf and I are now engaged. I know this may sound terribly shallow of me but her younger sister of age 15 is so clumsy. I try not to get involved with matters that don't concern me but it agitates me. I have bought my gf many things, including televisions, ipods, computer software, exercise machines. Everyone one of these items somehow ends up broken by her sister. She is such a klutz. Recently, she tried plugging in a tv I gave my gf, and she couldn't reach the plug, she tugged on the cord and it fell down and won't work. But when confronted she claims she has no clue as how it doesn't work, etc. It bugs me even more, because my gf is always claiming that her sister will be living with us when we get married, because her own parents fight at times. I dont wan't this. Yet, she gets mad and claims I am trying to forcefully make her alienated from her sister. So help me Plz!?