I think one of the most important jobs I had as a mom was to teach my children that life wasn't fair and that railing against the things we saw as unfair only made us miserable and petty.
My kids competed in the same sport, and when one would do well enough to earn an invitation to a national competition and the other one didn't, it was devastating for the one that didn't. So we comforted the one the didn't, cheered the one that did and had private conversations with both of them about the importance of considering their sibling's feelings. We taught the winning sibling to express sympathy to the one who didn't and scolded the losing sibling if they behaved churlishly due to their disappointment.
This approach might work for you since it seems a similar dynamic.
