The fact that your husband has no memories but his abuser/brother does says that he may hold the key for helping your husband. Since you already confronted your brother-in-law and he gave you info that your husband doesn't have, he is in a position to help your husband and maybe help the marriage.

Why not ask his permission to bring up the topic with the therapist, either in a joint session with your husband and the therapist, or allow the therapist to call you for this information?