What is the ruling on a "badal" marriage (exchange marriage) in which marrying a sister is stipulated in exchange for marrying a brother? Is this considered the forbidden "shighar" marriage? And what should the children do and how should they act with their parents in this situation?

"Shighar" (exchange marriage) is absolutely forbidden, even if a mahr (dowry) is specified in it. Therefore, the condition imposed by the girl's father, that his son marry the suitor's sister, is impermissible and falls under the forbidden "Nikah al-Shighar". If the guardian refuses to marry his ward to a suitable man of good religious commitment and character, the guardianship transfers to the next closest male relative in the agnatic lineage, then to the Shar'i judge. It is advisable to refer the matter to the Sharia court to determine if "adhl" (unjust withholding of marriage) has occurred.