What is the ruling on a brother who prevents his forty-year-old divorced sister from traveling, even if it leads to estrangement?

If a sister travels without a mahram (unmarriageable kin) or if there is a fear of fitna (strife/temptation) concerning her, then preventing her is correct, based on the Prophet's (peace and blessings be upon him) saying: "It is not lawful for a woman who believes in Allah and the Last Day to travel a distance of a day and a night unless she is accompanied by a mahram." She should be dealt with wisely, and the danger of unlawful travel and the lack of blessings that result from it should be explained to her.