Can other family members challenge a sale to one sibling after the fact?

A properly documented, genuine sale is generally difficult for other family members to unwind after the fact simply because they're unhappy with it. Successful challenges usually turn on specific problems with the transaction itself — that the parent lacked the capacity to understand what they were agreeing to, was subjected to undue influence or pressure by the purchasing sibling, or that the "sale" wasn't a real transaction at all, with no genuine payment, no independent advice, and no proper documentation, suggesting it was really a disguised attempt to benefit one child unfairly.

This is exactly why an independent valuation, separate legal advice for the parent and the purchasing child, and clear records showing the parent's intentions at the time are so valuable — they create evidence that the transaction was legitimate if it's ever questioned. If you're concerned about pushback from other family members, put these protections in place from the start rather than treating them as optional formalities for a family deal.