Is embezzling money from my brother's store considered theft, given the unpaid help I provided, and what is the expiation for this mistake?

Since you took this money secretly from its rightful place without your brother's knowledge, and you have no claim to it, nor was it entrusted to you, this is considered theft. You must repent to Allah Almighty for this sin. One of the conditions of repentance is to return rights to their owners. Therefore, you must return the money you took to your brother or seek his forgiveness for it, based on the Prophet's (peace and blessings be upon him) saying: "It is not permissible for a person to take anything from his brother's property unless he gives it willingly." As for your work with him, since you volunteered to help him, you are not owed anything by him. However, you can agree with him on a specific wage for your future work.