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?