When I was a teenager, I used to take money from my father and my older brother without their knowledge. Now I regret what I did, and I am wondering whether I should return the money to its owners, but I do not know exactly how much I took.
Praise be to Allaah.
What you did comes under the heading of stealing, but you regret what you have done and the Prophet (peace and blessings of Allaah be upon him) said: Regret is repentance. Narrated by Ahmad, 3558; Ibn Maajah (4252; classed as saheeh by al-Albaani in Saheeh Ibn Maajah. What is meant is that regret is the major part of repentance.
Praise be to Allaah that you have been guided to repent. Part of your repentance is returning things to their rightful owners, or asking them to forgive you and let you off.
You have to figure out how much you took, and do your best to work it out, then return it to your father and brother.
You do not have to tell them that you had stolen this money from them, rather the point is to return the money to its rightful owners in whatever way possible.
See also questions no. 31234 and 40157.
If you want to tell them what happened and ask for their forgiveness, this will not harm you, in sha Allaah, especially since you did this when you were a teenager and Allaah has now enabled you to repent from it. The Prophet (peace and blessings of Allaah be upon him) said: The one who repents is like one who did not sin. Narrated by Ibn Maajah, 4250; classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
You must hasten to do that, because the Prophet (peace and blessings of Allaah be upon him) said to his companions one day:
Do you know who is the one who is bankrupt? They said: Among us, the one who is bankrupt is the one who has no dirhams and no goods. He said: Rather the one who is bankrupt amongst my ummah is the one who will come on the Day of Resurrection with prayer, fasting and zakaah, but he will come having insulted this one, slandered that one, consumed the wealth of this one, shed the blood of that one and beaten this one, all of whom will be given some of his hasanaat (good deeds), and if his hasanaat run out before the scores have been settled, some of their sins will be taken and thrown onto him, then he will be cast into the Fire. Narrated by Muslim, 2581.
We ask Allaah to accept your repentance, and to help you to say and do the right thing.
And Allaah knows best.