ELF2588, here's an idea for you. (Call your vet first and get his/her okay!)

How about brushing something on Mimi's nipples that Gracie doesn't like. Citrus juice might be a good choice. Not toxic, but lots of cats don't like the smell. It might be enough to deter Gracie ... though I know those suckling instincts are very powerful.

Good luck, and keep us posted!

Dianne