3 groomsmen and no bridesmaids = bad idea.
When is he giving this speech, before the wedding or at the reception?
Is he paying for the wedding or giving his sister away in the ceremony?
He sounds like an immature kid who wants to be a part of the show, imo.
If he's not paying for anything, you and your fiance need to tell him to be like the rest of the guests and sit his ass down!