All Slivers have ", Sacrifice this permanent: This permanent deals 1 damage to target creature or player."
All Slivers have ", Sacrifice this permanent: Prevent the next 1 damage that would be dealt to target Sliver creature or player this turn."
2/1/2007 Note that the first ability can affect any kind of creature, but the second ability can affect only Slivers. Both abilities can affect players.
7/1/2013 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.
7/1/2013 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.