The latest episode somewhat supports your theory about it being because of her sister’s death, but I’d like to propose another theory.
So far, we have not gotten much of an idea of what Yusa thinks about this entire situation. Against her own will, she’s been transferred to a new school and forced to join the student council. She had absolutely no say in this and is pretty much being dragged along for the ride. She doesn’t really have much control over her life really. All these decisions were made without much argument from her. When Misa takes over, she has no control at all and all her actions are taken over by another person, so I think that her ability reflects that part of Yusa.