Does killing Might Sliver before damage affect other slivers' power and toughness?
The ability from Might Sliver is a static ability: it functions as long as Might Sliver is on the battlefield.  When the sliver leaves the battlefield (dies in this case), its ability also goes with it.  Your other creatures loose the power/toughness increase. 

At the time that your opponent played the instant, it was still in the Declare Blockers step.  During that step, you declare blockers, then both players have a chance to play instants before combat damage. Once your Might Sliver is dead, your other Slivers are 1/1 when you choose to move to damage. 


  604.2. Static abilities create continuous effects, some of which are prevention effects or replacement effects. These effects are active as long as the permanent with the ability remains on the battlefield and has the ability, or as long as the object with the ability remains in the appropriate zone, as described in rule 112.6.