D only really trusts two people--Elena and MC. She wouldn't believe either would betray her. Unfortunately, D will soon change her mind about Elena and possibly MC [this latter depending on game-paths chosen by gamer]. Remember, D has been made recently aware that the most important person in her life before getting together again with MC, her mom, has betrayed her for Brad's business interests.
Besides the romantic/sex relationship D has with MC, he is now viewed as her only family. Older mentors--Georgina, Olivia and Koko--are fine with D until they try to get too close to MC. It would be a contrived stretch to believe that D would wish to intimately share MC with anyone else.