"I'm only grandfathering you in because of Serena."

Solution 1:

Piskvor offered the relevant link, which can tell you more about the history of the expression. "Grandfathering" means that an exception is made because something existed before the rules were changed. I don't know what the context is in the example you've given, but here's an example:

My company recently began requiring us to write our documentation in Google docs. My current project is grandfathered; I can still use the old file sharing system because that's what I used when I started documenting, before the change was made.

I hope that clears things up.

Oh, and I think "grandfathering you in" sounds odd. This doesn't strike me as a normal way to use the phrase.

Solution 2:

A grandfather clause is a rule allowing currently running arrangements to continue despite new regulations.