When my friend Joan got married, she decided she would not have any attendants. She deliberately chose to go this route, excluding her own sister (about her age), in order to avoid including the groom's two sisters, whom she didn't like.
Joan also has a much-younger (half-) sister from her dad and stepmom, who was about 5 at the time. The stepmom told her daughter that she could be Joan's flower girl, and got her all excited about the pretty dress and so forth.
Joan, not wanting to make waves, decided to allow it (and apologized to her adult sister, because now she was including one sister and not the other).
At the wedding, the stepmom also produced a small boy in a little tux--some relative of hers who had been invited with his parents--and put him in the procession as the ring-bearer. I asked Joan, "Who's that?" and she's like, "Sigh. I don't know." The kids were well-behaved and looked cute, so I suppose all's well that ends well, but I would have been pretty mad if someone pulled either of those on me.