I would surmise that your T was referencing the boundaires associated within the sibling connection/relationship and those that relate to sharing personal information that reference other individuals. Both are real and effective boundaires for maintaining unconflicted relationships.

I would ask B if it is possible that A tell you herself. If not,,it is an issue within the marriage that seems to reference A and it would be a form of intrusion for B to share it.(boundary).

IMHO.

Lenny