Just about any 2 bedroom unit can handle 6 people as most have sleep sofas. The occupancy is restricted by condo rules and owner rules.
The main places will be pretty booked, but check the following:
Sanibel Arms, Sanibel Arms West, Sanibel Moorings, Sandalfoot, Ocean's Reach, Sanibel Siesta, and just off beach but just a walk over Clam Bayou to the beach, Blind Pass up by Bowman's Beach.
For units in other complexes, check any of the agencies, including Royal SHell, Select Vacation properties, Kingfisher, VIP - the main ones on Sanibel.
Some complexes to look at:
Island Beach Club
Remember, most places will only take Sat-Sat online reservations (except time share places like Tortuga Beach which are Fri-Fri), so if you look for other than those dates, only a few of the first places I named might allow online reservations. Otherwise, you'll have to call the agency to see if you do anything. It's a heavily booked week, so try Sanibel Moorings and Blind Pass Condos early as they may well be the ones with most availability, followed by Sanibel Arms West, Sandalfoot and Sanibel Siesta.