My sister offered her drink to my boyfriend AIO

The three of us were together in a pub, (I don't drink alcohol) and my sister and my boyfriend were both drinking alcoholic cocktails. My sister couldn't finish hers and asked my boyfriend if he wanted it, considering it a waste if she threw it away. My boyfriend agreed and I got angry, it's an intimate thing to drink from someone else's glass and I don't understand why my sister shouldn't have just left the cocktail there. We ended up arguing and I walked my sister home. She later said that the straws were different and that's why she thought it wouldn't bother me. AIO?