What is the way to repent and reform oneself after stealing money from one's mother and sister, not knowing the exact amount, and after the sister's anger and estrangement?

The questioner's remorse and repentance may be accepted by God, but repentance from sins that involve the rights of others requires seeking absolution from the rights-holder or restoring their rights to them. If your mother and sister absolve you of what you took from them, then your conscience is cleared. Otherwise, you must return what you took, and if you forget the amount, then strive to return what you believe, with reasonable certainty, will clear your conscience, and ask for their forgiveness.

It is obligatory to mend the relationship with your sister and remind her of the sacredness of maintaining kinship ties. If she refuses, you must continue to connect with her and treat her kindly. The answer advised increasing acts of seeking forgiveness (istighfar), good deeds (hasanat), associating with righteous people, constant remembrance of God (dhikr), supplication (du'a), and contemplating death.