You need to have enslaved Sarah for Cass, either by dating Sarah as Dan and then getting a scene where Sarah confronts you in the streets about Cass's control over you and choosing to side with your mistress, or enslaving her as Rosa, putting her in Mindless personality, and doing the Library play with scene with Sarah, then have 3 or more slaves of your own, for Val to not be one of those 3, have had the prompt to give up one of your slaves, have given up one of those slaves, and then getting Cass to ~level 20 and encountering a scene in your bedroom.