Hmmm... I beg to differ on this one. Have you considered your sister feels like she has to compete with you? Were you encouraged to compete with each other as children? Did you get something she prized?
I wouldn't run a test like that on a man. It doesn't prove anything because we just know the bit of what you told us like is she a knockout and you are a plain jane, did she pour it on thick or just showed more interest in him as a man than you did as a college buddy. We don't know and we can't judge.
But what of your relationship with your family as it may well be not just you and your sister?
I had 2 sisters trying to speak for my late husband and I told them where to go and too his attention back to me but then he was in love with me so I didn't have much problem.
You see the difference is I was in love with him and him with me. How you have explained it sounded like just an interest but I recall my sister trying that with me when we were young. She pointed out this guy and said, I bet I could get him before you do so I told her to go for it as her interests were definitely not my interests and besides that I was too young by my standards to be tied down with a boyfriend. Most teenagers have different ideas then settling down which was me.
All relationships are important and should be handled as God would have as handle them. We are all important to Him as He created each and everyone of us in our mother's womb