The sand sifting star is going to eat all the things in your sand bed that help with the clean up and then most likely starve to death. They need large tanks and lots of sand in order to survive.
What kind of snails do you have?
An astrea or two, 3-4 nassarius snails(The really small species), and some bristle worms will probably provide plenty of clean up for a 12 gallon without the worry of them starving each other and polluting your tank.