How do I get Rinny to teach me the cock growing spell? When I talk to her she always says "Hmm... maybe later, ok?"
Have you Visited the Tall Vampire Lady under the Manor? Or Edith, for short.
Ask Pixie, about her and she will give you "Do what you want" ticket. Go to the Basement, and Bam.
Ensure that you Bring Rinny, along for this or you can't do anything to Edith.
Make Rinny, do the spell on her and she will gladly teach it afterwards.
The three requirements to get that spell are:
1.) Have Mystic stat at 7+
2.) Rescue Rinny from the manor
3.) Watch Rinny grow a cock at least once
For that last one, you actually have a few options. As said, you can bring Rinny with you to get revenge on Edith. But also, if you have a vagina, you can go in for a hypnosis session, distract Rinny by poking her, and then fail to resist the hypnosis (on purpose or not). Lastly, you can say "No" to everything Rinny offers to do to you when you visit the first time, per trip to Transylvania, then pick "have her grow a penis" from the bottom of the list. For that particular option, you don't even need to finish the manor first.