YTA so very clearly. I won't here anything about "where I'm from we don't ever leave our children alone" until you specify where that is. A 2 year old is more than capable of being left alone for a moment while you get a drink and if your mom has your daughter so often some sort of baby proofing should be done as it is not possible to be with the child 24/7. On top of that I would never leave my daughter with someone who straight up told me there were uncomfortable watching her, that was so irresponsible on your part. To cement the YTA, calling your sister dumb for leaving the child alone is completely uncalled for. You were a child who had a child, I'd refrain from calling others dumb.