At most two mana of any two different colors available, plus enough mana to pay for commander tax, if applicable.
Steps
Activate all Slivers you control by tapping them, adding one mana of any color for each Sliver tapped this way.
Cast Sliver Grandmother from the command zone by paying ({W} magic symbol) ({U} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) and commander tax, if applicable.
When Sliver Grandmother enters the battlefield, Intruder Alarm triggers.
Holding priortiy, activate Sliver Gravemother by tapping it, adding one mana of any color.
Resolve the Intruder Alarm trigger, untapping all creatures.
Activate all Slivers you control by tapping them, adding one mana of any color for each Sliver tapped this way.