Activity for Jillian

So we are having an after party immediately following our reception. It's an hour and a half long. My fiance thinks a note about it should be on the invitation itself, I think it should be a separate insert. We don't even know how to word it at all....

I've always wanted a fairly small wedding party but my fiancé has two sisters who I'm sure would like to be involved. Am I obligated to ask them to be bridesmaids? Aside from my sister, I don't even have any of my own family in the party. I don't...