Lens or Flash?

I currently have the 7D along with the EF-S 17-55 2.8 and the EF 70-200L 2.8 MKII. As a new Grandpa, I mostly use the camera for family events, where lighting is poor, subjects are moving and in most cases flash is not allowed. I have been saving for an additional lens, EF-S 10-22, as I have found myself in some of these family situations where simply stepping back still does not allow me to frame the shot.

I have recently been asked to shoot a wedding rehearsal dinner, bridal party gathering (before the wedding shoot with bride and her bridesmaids in a hotel suite) and the wedding reception. Seems their budget allowed for the wedding, but no other events.

Question for the forum, should I consider the 580EX II flash (and start practicing), the 10-22 lens or something else? I have about $800 and want to make an investment that will benefit this type of shooting leveraging the equipment I already own.