Was it correct to inform the brother about his teenage sister who was communicating with young men online, which led to parental reprimand and the brother being accused of stirring up discord, and what is the ideal course of action in such a situation?

You have done well by denouncing your sister's conversations with foreign men, for this is a reprehensible act and a gateway to corruption. A foreign woman's conversation with a foreign man is not permissible except out of necessity and within the bounds of Islamic regulations.

There is no harm in informing your brother if you believe that it will lead to her reform, and you will not be considered a slanderer for doing so.

If your father is alive, he is responsible for her, and he must restrain her and prevent her from this reprehensible act. It is incumbent to continue advising her and her guardian with wisdom and good counsel.