I've been reading some fluff fics and I just remembered this one. Searched around the comms but no luck so far.

What I remember:

J2 established relationship. Jared as "Hannah" (I don't remember his popstar name). Jensen's sister is a fan of "Hannah Montana" and sings his song and Jared unconsciously sings along, I think she finds the secret out early on. I think one of J2's friends is also a fan. Jensen finds out eventually.

Thanks in advance for any kind souls out there!

Edit: Found in comments!!!
Thanks to goldenboat!!!