Falling out over not buying drinks

I went to a family party a few weekends ago. I arrived at the party with my parents, girlfriend and little sister. As soon as we arrived my dad bought us all a round, about 15 minutes later just as we were finishing our drinks my other sister and her boyfriend arrived and they got themselves a drink. About two minutes later my dad asked everyone who wanted a drink, my other sister had gone to the toilet but her boyfriend said to my dad "we're alright as we've still got plenty." So I went to the bar with my dad to give him a hand carrying the drinks but I ended up buying the round instead. After that my sister and her boyfriend got themselves another drink each and then left after that, they were only at the party about an hour.

Fast forward to last weekend and my mum asked me if I bought my sister and boyfriend a drink at the party, I said no and explained why and then asked why she'd asked that question. She then told me that my sister was upset that I hadn't bought them a drink. To me it just seems childish to get upset over such a thing when firstly I asked her boyfriend if they wanted another drink and he said no and then they left the party after only an hour and didn't buy any of us a drink.

Anyway I tried to speak to my sister about it and she called me selfish to not buy them a drink when it was my round. So again I explained to her that firstly I asked her boyfriend if they were alright for drinks whilst she was in the toilet and secondly it was never intended to originally be my round to buy, I only bought them during a spur of the moment thing. To which her reply was, I should have asked her when she came back from the toilet if she wanted a drink.

I have distanced myself from my sister now, I think that falling out over drinks when I have explained what went on is pathetic so if she wants to sulk over me not buying her a lemonade then that's fine.

Should I just leave her to it?