As the muscle cords of the creature twitched, Hanna saw an unsettling unanimity in the others' rippling flesh. She didn't know what it meant, but she urged Sisay to keep the ship at a safe distance.

Sinew Sliver

Commander Masters

Time Spiral Remastered

The List

Planar Chaos

Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn't provide any additional benefit.

If the creature type of a Sliver changes so it's no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a Sliver may become lethal if Sinew Sliver leaves the battlefield during that turn.

Sinew Sliver's ability does give itself +1/+1.