What detailed guidance can be offered to someone who has reached forty and is not yet married, despite his efforts and keenness, and his service to his sick mother and sisters? And is this delay due to a shortcoming on his part?

Your righteous work in caring for your mother and siblings is great, but it should not delay marriage, especially given your need for it, as marriage aids in matters of both religion and worldly life. If you complain of difficulties in your affairs, then examine your deeds and your relationship with your Lord, because difficulties in affairs and closed doors may be a result of sins and shortcomings in fulfilling your duty to Allah, as mentioned by Ibn Al-Qayyim and Ibn Taymiyyah. Therefore, you must fear Allah and seek His forgiveness; for by them, the benefits of both worlds are attracted, problems are solved, and provision comes from where you do not expect.