I think if you are truly torn between this you should decided who is more important, Jim and Sterling or the foster kittens
First I'd like to say that I will not put one group ABOVE the other group. Of course, my cats are MY responsibility, but I agreed to take on the fosters and they TOO are my responsibility. EQUALLY.

with that said......

I don't think the kitty Prozac is the answer to everything and personally I would try to wean them off of it and keep the kittens away. If they start spraying again without the meds then you know something besides the kittens is bothering them.
The words in the above quote "WEAN THEM OFF IT" made me realize that this IS hopefully what will exentually happen. But what I forgot to add into the equation is that someday there will be more and/or different fosters. And then we have the same problem all over again.

This is why I asked for all different points of view. Even though I try hard to look at all the angles, sometimes there is something I forget to think about.