Let the younger one continue it. The older sis maybe upset, but it's not fair to the younger sibling to miss out because of that. My mom did that to me, I was invited to a program for gifted children, but because my sister was never in it she didn't let me. I'm still mad about it today...ok, not really, but I do remember it. Now that we are adults my sister was ticked when she found out about it. My mom said she didn't want to hurt my sisters feelings, my sisters response. "So you hurt J.'s instead?"
Don't think you can hide it either, my mom just didn't respond, and when school started my teacher asked me why I wasn't in the program. Ooops teacher tattled. :)