How can I act regarding money that I took from my father and brother without their knowledge in the past, given my regret for that and my inability to recall the exact amount?

What you did was theft, and your remorse is repentance. You must return the money you took from your father and brother. It is not a prerequisite to inform them; it is sufficient for the right to reach them. If you wish to inform them and ask for forgiveness, there is no harm, for "the one who repents from a sin is like one who has no sin." You must hasten to do this before the Day of Resurrection, when the bankrupt person will come, having taken people's rights. Then, his good deeds will be taken from him. If his good deeds are exhausted, then some of their sins will be taken and cast upon him, and then he will be cast into the Fire.