Is one who allowed a woman—whom the deceased had instructed not to participate in her ritual washing—to enter and perform the washing, sinful?

We have not found any statements from scholars regarding this issue. However, it appears—and Allah knows best—that if the will to prevent a specific woman from washing the deceased was for a legitimate religious reason (e.g., she does not perform the washing well or is not trustworthy), then it should have been adhered to. Violating it would require repentance and seeking forgiveness for the deceased. But if the will was not for a legitimate religious reason and would lead to corruption, such as severing family ties, then it should not be enforced, and the washing is valid in any case.