Sister conflict

So I've liked this guy for about 2 years straight  and one day my sister asked me if I still liked him, I said no trying get over him because my crushes never like me back, or I lose them to them liking my sister. Her teasing me about liking him for the past few months I thought it was just a question. And now they've been dating for 2 months 😞 My whole family has known that I've liked him even teased me about it but now there's a big elephant in the room when she has him over. I've tried to be the bigger person and get over it and I was progressing in that direction until she went to prom with him a few days ago. My heart sank into my stomach as I was getting her ready and the look on his face when her saw her made me feel even worse. Part of me want to keep trying to be happy for them but the other half hurts because I always lose my crushes to her. What should I do? 

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