You may choose not to untap Rubinia Soulsinger during your untap step.
: Gain control of target creature for as long as you control Rubinia and Rubinia remains tapped.
10/1/2009 If Rubinia Soulsinger leaves the battlefield, you no longer control it, so the duration of its control-change effect ends.
10/1/2009 If Rubinia Soulsinger stops being tapped before its ability resolves — even if it becomes tapped again right away — you won’t gain control of the targeted creature at all. The same is true if you lose control of Rubinia Soulsinger before the ability resolves.
10/1/2009 Once the ability resolves, it doesn’t care whether the targeted permanent remains a legal target for the ability, only that it’s on the battlefield. The effect will continue to apply to it even if it gains shroud, stops being a creature, or would no longer be a legal target for any other reason.