This question already has an answer here:
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 Sliver has a static ability: "All Slivers get +2/+2", so in fact I have two (3/3)'s and one (4/4) creature to block with.
If, 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 three creatures, while he loses two, or is it lost and my defense crumbles and my defenders are all killed?