Okay, I'm gonna be blunt here. Please do not get mad. I'm just trying to show you how I see it and a possible option.

It's obvious that Anika has been with you far longer than Sebastian. It's not fair that Anika has to be medicated. She was there first.

Here's why I say this:

I had adopted a cat from a NYC kill shelter. He was 6 months old. I thought for sure it would work out. Hardly. Tucker, nka Otto, kept sabotaging my Casey (RB) in the litter box.

The vet gave me a sedative for Casey. It turned him into a totally different cat, always hiding in the closet. That's when I had to make the most difficult but correct decision of my life, to re-home Tucker. Best decision I ever made as he is now spoiled rotten and I have visitation!!

You do what's in your heart and what is best for the new kitty.