Is it permissible for a mother to keep her extravagant son's money without his knowledge in order to get him married, and is she obliged to inform him of this before she travels for Umrah, and is her action considered a sin?

If your brother is a spendthrift and squanders his money on what Allah has forbidden, then it is permissible for his mother to take some of his money to reform him and prevent him from extravagance or spending it on unlawful things, based on the hadith: "Whoever can benefit his brother, let him do so." She must also take the necessary measures to safeguard the money from loss, such as documenting it or writing a will concerning it.