i got quite confused on what exactly happens in this situation:

Opponent is attacking, he does so with three (3/3) creatures.
I declare blockers, which would be two unimportant (1/1) Slivers and a Might sliver (2/2), the Might Slivers effect is static ability: "All Slivers get +2/+2", so in fact i got two (3/3) and one (4/4) creature to block with. If now, after the blockers are declared and before the damage is dealt, the opponent destroys the Might Sliver with an instant (Murder), will my Slivers retain the +2/+2 bonus until damage is dealt, so i would lose two creatures, while he loses all three, or is it lost and my defence crumbles and defenders are all killed?