Whenever a Sliver becomes blocked, that Sliver gets +1/+1 until end of turn for each creature blocking it.
10/4/2004 If two Spined Slivers are on the battlefield then each blocked sliver gets +2/+2 for each creature blocking it.
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.