Is it permissible, in Islamic law, for a woman observing the 'iddah (waiting period) after her husband's death not to visit her sick brother, who later passed away, even though there was no dispute between them and her family urged her to visit him?

A woman whose husband has died must remain in her home for four months and ten days. She is not permitted to go out except for a necessity during the day. Visiting the sick is not considered one of these necessities. Therefore, what this woman did by not going out is correct and in accordance with Islamic law.