It all depends on your price range. You could go to a dollar store and make her a vase- I made one for a girl in my chapter. A tshirt with the Sigma letters on it would be cool. Like ihearttrisigma said
www.alphahouse.com has a lot of really cute Sigma items and it isn't overpriced. Check them out! Also, think of what you like that is Phi Mu related and get your cousin something that is similar for Tri Sigma. Good luck!