confirmMembershipSibling() always called nextStep(). With a single-child
wizard that was the only thing it could have been resuming, but the warning is
now reachable from "add another sibling" too — and answering it there jumped
the desk to step 2, throwing away the child they were half way through adding.
It now resumes whichever action raised it.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>