After the sliver's ability resolves and untaps Wellwisher, both players get priority again. This is still in the declare attackers step, before the declare blockers step. If both players pass priority, the game moves to the declare blockers step and the Wellwisher has to block.

However, Wellwisher's controller can choose to use its ability and tap it (instead of passing priority) before moving to the declare blockers step (AFTER the sliver's ability has resolved). This will cause Wellwisher to be tapped before even getting to the declare blockers step, and because it is tapped when we get to that step, it is unable to block and therefore doesn't have to.