Being XO doesn't mean you're next in line for the Captain's chair of your current assignment. Sisko is an experienced commander and war hero. In a real life military that would make him a top choice for the fleet's flagship. Kira may be a longer shot. Looking at Kira Lit history on Memory Beta, I see she is also an experienced commander and held the rank of Captain in Starfleet. So again, a real life military would see her as a viable commander for the Enterprise.