I think a very loud, incredulous "YOU'VE GOT TO BE KIDDING ME.... US TOO!!!!" from your cousin would have been the appropriate response. With much laughter from everyone to follow, turning it into an extra special celebration. You're cousin seems to be living in a very small world, she needs to wake the heck up.
Wow, this has ME really upset, poor kid. As a grandma myself, you better know I'd do whatever, give my own share to one of my grandkids so they weren't left out. Even my grown kids, it's just what you do. Aww, I could just cry