Dear Michael: What's a pencil skirt?

Dear Michael,

I'm getting married and have invited a few out-of-state guests to the wedding. My mom said I have to invite them to the rehearsal dinner. I don't know how to present that idea to my mother-in-law, since she's paying for the dinner. What is the proper etiquette, and how should I discuss this with my in-laws?

Signed,

The Perfect Bride