Why do brothers prevent their sister from performing Hajj or Umrah despite her strong desire and her fear of it being too late?

Hajj and Umrah are not obligatory for a woman as long as she does not find a mahram (unmarriageable kin) to accompany her. If she dies before being able to perform them, she will not be held accountable, as she was incapacitated. Relatives are advised to help her perform Hajj and Umrah if they are able, and it is recommended for them to accompany her if she provides for their expenses. Some jurists even consider this obligatory. Some scholars have permitted a woman to travel for Hajj without a mahram if she is with a safe and trusted company. It is also advised to make abundant supplication for the facilitation of the matter.