Mine just happened today.
Me and my sister her husband live with my parents (due to financial issues), I Have my upstairs bedroom and my sister, my brother-in-law and their two very young daughters (1 and 4) have the whole basement to their selves. Today, my sister caught the flu and she needed me to watch the kids while my brother-in-law was at work.
So, I'm down stairs watching Beauty and the Beast with my nieces in their "living room" when the youngest about 14 months old got into their mini fridge(which is very EASILY accessible to both of these kids) and saw her playing with a bottle of beer. I was reminded when I was volunteering in the ER at hospital last year when this kid about 3 almost died from alcohol poisoning when he drank his dad's roman coke. I freaked out, and grabbed the beers and brought them to my dad.
Another thing that is kind of relevant, we live in a conservative Mormon household so my mom, dad, and me has this thing about alcohol, we don't drink it and we don't want it in the house. When they moved in, my dad told my sister and brother-in-law that it is okay to drink when they're out but not to drink and keep any booze in the house. I don't care what your personal beliefs are, but I am very uncomfortable having any here in the house, especially when small children live here.
Anyway, looking back I would have done some things different, like not going to dad about it and confronting my sister first, but either way it was going to end and it did end with the disposal of the alcohol.
My dad wasn't going to tell mom about it (since she already has more than enough on her plate atm), and they just got a warning from my dad and a request not to bring anymore home. I feel like I did the right thing, but when my sister found out she punched me in the head, and being even the same room with my brother-in-law it's extremely uncomfortable and I can see murder in his eyes when he looks at me.