What is the ruling on a sister being lenient in speaking with non-mahram young men, on the pretext that they are her brothers or are married, and her joking and laughing with them? Is this action haram, and what is your advice to her?

The easygoing conversation of young men opens the door to fitna (temptation/strife) and corruption. Your sister must sever her ties with them in obedience to her Lord and to protect her honor. Her excuse that they are younger or like her brothers is invalid, for Allah addressed the Muslims, saying: "And when you ask [his wives] for something, ask them from behind a partition. That is purer for your hearts and their hearts." (Qur’an 33:53). A man's conversation with a non-mahram woman without necessity is impermissible, as it is a cause for fitna.