Breast Milk Thief

Ok,so weird. So earlier today I was at my sisters house visiting. Everyone was in the living room and I decided to go to the kitchen for a snack. When I walk in my brother in law is in the pantry drinking what I thought was a Coke but when he turned around he was drinking my sister's stored breast milk for the baby. I played it off as if I didn't see anything,uhg so weird! An hour goes by and my sister opens the fridge and said "that's weird I thought their was another bottle". Her husband didn't say a word!!! What should I do,it's so uncomfortable. I'm afraid if I say somthing she won't believe me and I'm going to end up looking crazy!!!