If you really want your sisters up there with you (and I can see why!) I would suggest this.... maybe keep the 2 groomsmen as is, if both sisters make it - perfect it's even, if not then do what was suggested above and have the 2 groomsmen walk your sister down the isle. I think that would look lovely!
My fiance ran out immediately and asked 3 men to stand up for him so I was left to find 3. I wanted even numbers as I wanted them to walk down the isle together. I also waited until I knew who was coming to the wedding to ask them as they didn't have any pressure to commit.
I helped with the dresses and I got them all their supplies for the wedding day (jewelry, sandals, etc.) they did pay for their own hair and makeup though at their discretion, one had considered doing her own but then changed her mind.
I gave them ample notice of the cost of of these extras though, so they knew if they could commit or not. While we don't want to place a large burden on people, I think you may be surprised that most don't mind. It's an honor to them and something we may have done for them too, etc. Of course though, with that said, I made sure to keep things within reason!!! Aside from their trip, my bridesmaids paid about $250 for a dress and their hair/makeup.
Edited by calgarybride2015, 10 March 2015 - 06:23 PM.