Boyfriends giving alcohol

Hi! First i am 19 and underage. I was at my bfs parents beach house for the day. They offered me a wine cooler and i declined because his 12 year old brother was there and i just didnt think that was right or a good example for him. So his mom grabs a black cup and pours it in there instead. I had tiny sips so i didnt seem rude but my bf didnt care. My sister is 10 and i wouldnt want my bf drinking in front of her until it is legal. Whats your opinions?