celebrity by proxy

Close relative to the professional celebrity you are famous because you are related(not quite) to someone famous.

The future(?) son-in-law of the former presidential candidate has become famous as a celebrity by proxy.

celebration by proxy

Something one does to treat themselves, due to a celebration going on for someone they are close with.

“It’s my roommate’s birthday tomorrow, so I’m having a cheat day as my celebration by proxy.”