Is it permissible to deduct 2.5% of the weekly wage to pay Zakat monthly, and is it necessary to ensure the sufficiency of the needs of the mother and financially dependent siblings before paying Zakat, or is there a way to combine both?

It is permissible to prepay zakat if the salary reaches the nisab (the minimum amount on which zakat is due). If the salary does not reach the nisab, then there is no zakat on it.

Caring for one's mother and siblings takes precedence over prepaying zakat, due to what Muslim (997) narrated from Jabir, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "Begin with yourself and give Sadaqah to it. If anything remains, then for your family. If anything remains after your family, then for your relatives. If anything remains after your relatives, then like this and like this, in front of you, and to your right, and to your left."

It is preferable to first attend to the needs of the family. Then, if wealth remains that reaches the nisab, prepaying zakat is permissible, but delaying it until the completion of the hawl (one lunar year) is better. One can also give Sadaqah (voluntary charity) as much as is feasible after the family's needs are met.