Unless if you find something spacificaly wrong with him you do need to put on an act for your sister's happiness. I think without realizing, you are jealous, and its okay to be its just human nature! Inshallah you'll find someone also, but until that time you will feel this for a while. Don't take it so hard on yourself, but think of it in the sense, you wouldn't be happy if he was treating her bad, so just thank god that she found someone good!
Also since your the older sister, you will feel like this, its a given, because thinking in numbers you should get married first, but its not your time yet! And remember, ALLAH does not like jealously, so try to refrain from it. :)