I came back from college to find my sister had gotten her license. My Honda Accord became OUR Honda Accord. My sister has attached numerous fairly heavy key chains to the car keys. I told her to take them off for it would ruin the ignition with all that weight. She told me to prove it.
You are right. A heavy key chain dangling from the ignition switch will cause it to wear out.
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Prove it? Short of letting her continue to use the heavy keychain and ruin the lock cylinder, you can’t.
But that’s what is likely to happen if she doesn’t, um, lighten up.
You’ve got more problems than a mere keychain. Resolve this with boxing gloves.
It will. Imports have gone to a side milled key not only for security reasons but they have a greater bearing surface so the 2 pounds of keys people put on the rings will not ruin the cylinder nearly as fast.
Be thankful it is your little sister not your wife. With a little sister the big college guy is in a better position to argue. 
Threaten her with not introducing her to all those handsome college friends you might know and might visit.
Yea, it can damage the lock cylinder over time.
How about this. Ask how much a new cylinder installed would cost and have your sister put up security equal to that.