It may have been very hard for B to come forward and speak to you. It sounds like his T is trying to get him to enlist some outside support in dealing with this. I would like to think that A's T would not suggest this unless it was appropriate. There are things you might definitely want to know about A whether A is part of this conversation or not. For example, maybe A has a substance abuse problem and B has tried all he could to get A to seek help with no success. Sometimes other family members are enlisted for a group "intervention" and you could really help here. Or perhaps A is abusing her children and you should know so you can call CPS. Sure, B should have taken that step but sometimes in marriages we have trouble confronting our spouses so B's cry for help should not be ignored. On the other hand, there are things B should not be telling you but handling in another way. For example--marital problems such as infidelity. If A is having affairs, there is no need for you to know that. B needs to handle how he feels about that and his action plan without consulting A's family members. His therapist can help.

I would have a discussion with B and tell him that there are certain things that are OK for you to tell me such as child abuse, substance abuse, etc. (you should come up with your own list). And there are certain things I don't need to know, and would harm my relationship with my sister for no good cause (boundary violation), such as marital fidelity problems, sexual problems, etc. (again, you make the list). Make sure B understands the difference between these two lists of things. Then offer to hear the problem if it doesn't fall in the second group of things. If it does, then don't hear the information from B and instead encourage him to seek support from his therapist and perhaps friends rather than A's family members. Or tell his own family members. Don't sacrifice your relationship with A simply to make B feel better. There needs to be a good reason you should know. Again, I am thinking there is a good reason or B's T would not have suggested this to him. But it is best to ask B directly, rather than depending on his T to have made the right judgment.

Good luck. At least B gave you a warning so you could make the decision.