Sturdy enough?

[beenthere]

I plan to build this shed on 4x6 skids, so i would like to steer clear of the pouring of concrete. What about sistering the floor joists?

Rather than "sistering" them, put them in every 6" on center. That will give more opportunity for them to stay dry and more rot-free. The blocking then will help them share the point loads. Planking over the plywood for runners under the tires will also help share the load.
Span tables and long-term loading limits will be important to include in the design. Are you putting in treated joists, and if so, what grade?

 

[Egon]

I plan to build this shed on 4x6 skids,

Just put in two extra skids. That means The joist will have support every four feet.