Should the brother responsible for the the heirs' money, who lost it in stocks, return all the money?

If your mother, aunt, and adult sisters permit you to manage and trade with their money, then you are their agent. As for your minor sisters, you manage their money in your capacity as their guardian. Neither the agent nor the guardian of an orphan's money guarantees against its loss or damage if the transaction was for the benefit of the principal or the orphan, and there was no transgression or negligence. However, if there was transgression or negligence, then compensation becomes obligatory. This was the fatwa of Sheikh Al-Islam Ibn Taymiyyah and other scholars such as Ibn Abd al-Barr, Malik, Al-Shafi'i, the Permanent Committee, and Sheikh Al-Shinqiti. In case of dispute, recourse should be made to people of expertise and trustworthiness to judge the matter.