Turn slow cooker to high and set timer for 2 hours. Pour in enough boiling water to come up one inch inside the ceramic insert. Fit a metal or bamboo steamer basket into the slow cooker and cover with slow cooker lid. Let water come back to a simmer, then add Brussels sprouts to basket. Cover and steam until tender, 15-20 minutes.
Meanwhile, in a large bowl, whisk together mustard, marmalade, and cider vinegar. When Brussels sprouts are tender, carefully remove steamer insert and transfer sprouts to bowl with mustard mixture. Stir until sprouts are completely coated and season with salt and pepper. Serve warm or at room temperature.