What should I do about taking money from my orphaned siblings and my grandmother who has Alzheimer's without their knowledge, and how do I repay these debts after my repentance, and should I tell my mother about it?

The repentant is given glad tidings of forgiveness and mercy from God, so long as the death rattle has not begun, or the sun has not risen from the West. God has promised the repentant forgiveness and mercy, for He Almighty said: (Your Lord has prescribed for Himself mercy. Indeed, any of you who does wrong in ignorance and then repents after that and corrects himself - then indeed, He is Forgiving and Merciful), and He Almighty said: (And whoever does a wrong or wrongs himself but then seeks forgiveness of Allah will find Allah Forgiving and Merciful).

As for the rights of human beings, these rights must be returned to their owners, or their permission must be sought to waive them. What you took from your grandmother must be returned to her, or her forgiveness sought if she is of sound mind. Otherwise, her right transfers to her heirs. Similarly, regarding the rights of your siblings, they must be returned to them or their forgiveness sought. If they forgive you and are old enough to do so, your liability is cleared. If they do not forgive, the rights must be returned, and you can inform your mother to help with repayment. If you are unable to repay and they do not forgive you, it remains a debt upon you until you are able to fulfill it, while striving diligently to do so.