Cousin Shelly? I don't think I've read that one, but it sounds like it matches that author.

The one FK use of the Freaky Friday trope that I always remember off the top of my head was a crossover between the original Nick Knight pilot and first-season Forever Knight, in which the two Nicks woke up in each other's realities. Many things happened, but the nicest that I recall were that our FK Nick got a lot out of talking frankly with Jack (the NK reality's Natalie) and the NK Nick fell in love with Natalie (our FK reality's Jack).

Happy holidays to you, too!