When my I'm babysitting the kids at my sister's house, I drink the milk straight from the carton

Blech! Urgh...bleah...URK!
I wish that was my sisters reaction instead if her screaming at me and making me go to the store to buy her a new milk carton. I guess it's Karma's way if getting me back though.
Have you thought about using a glass at her house?

Yes, but just to annoy my dear sister I drink from the carton anyway. It's worth it to watch her flip out momentarily even if she does scream at me and send me to the store.

Lol, wicked! I'll bet that's why she left you curdled milk that time! Teaching you a lesson!
FRAAHAHAHAHAHAHAHA! I remember that story!!!