Spined Sliver

• WWizards of the Coast2004-10-04
  Official Ruling
  If two Spined Slivers are on the battlefield then each blocked sliver gets +2/+2 for each creature blocking it.
• WWizards of the Coast2013-07-01
  Official Ruling
  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.
• WWizards of the Coast2013-07-01
  Official Ruling
  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.

Rulings via Scryfall