B. isn’t “wrong” for saying no. Your parents made a legal, adult decision when they adopted J.—that made them his parents in every way that matters, not you. What’s happening now is pressure and guilt being used to transfer responsibility because it’s convenient (or scary) for them as they age. It’s also concerning that they’re treating him like something that can be “reassigned” if you refuse—those emails and the “If B. refuses” note suggest they’re planning around their needs, not J.’s stability. You can care about his well-being without accepting an obligation you never agreed to, and you’re allowed to recognize that the relationship has been shaped for years as “siblings,” not “mother and son.”

What I’d do: hold a firm boundary and offer a structured, realistic alternative. For example: “I will not become his parent, but I will help plan his support.” That can include (1) meeting with a family lawyer/social worker to clarify what your parents can and can’t do legally, (2) putting a plan in writing for guardianship only if you genuinely want any role, and if not, identifying a stable long-term caregiver (trusted relative, family friend, or supported living options depending on his age/needs), (3) making sure J. has therapy/support because this situation can feel like rejection even if it isn’t, and (4) telling extended family: “If you think someone must take him in, you’re welcome to volunteer—otherwise please stop attacking me.” Boundaries aren’t abandonment; they’re honesty. The key is protecting J. from being treated like a contingency plan while protecting B. from being coerced into a life she didn’t choose.