I shouldn't have cited the APA like there's some specific line of "don't speculate", though it likely falls under a more general category. However, it clearly violates the "do no harm" clause. Deb was in an extremely vulnerable state, and said Dex was the only one she could count on or whatever she said. Being in a vulnerable and suggestive state, made to believe/realize she is in love with her brother (related or not, he's her brother) carries an ENORMOUS risk to her mental state...even if it's true. Additionally, the therapist suggests she confront the issue by telling Dex, would carry an extremely high risk of him not reciprocating (rejection) and likely distancing himself (loss of important support system), at a time when she is already vulnerable. Even if the therapist is right, which is a big IF (therapists aren't magical mind readers - modern psychology tends to avoid drawing such conclusions in favor of letting the patient come to those conclusions on his/her own), no therapist in his or her right mind would ever bring it up at that time (if ever).