Is it permissible to marry the fiancée's sister after the engagement of the older sister, reciting Al-Fatihah with a crowd of guests, announcing the engagement, and presenting the jewelry, then announcing her marriage to someone else?

If the engagement was merely a promise and no Sharia-compliant contract was concluded, then either party may annul it, but this should only be done for a legitimate reason. If the guardian annuls the engagement without informing the suitor, he has acted wrongly. And whoever proposes to and marries someone else's fiancée, knowing this, is sinful, because it is forbidden to propose to someone who is already engaged to his brother. The majority of jurists consider the marriage of a woman whose engagement was forbidden to be valid. It is not permissible for you to remain in a relationship with this girl, because she is now married. You must repent, cut off the relationship, and pray that Allah replaces her with someone better for you. If your heart is attached to her, there is a remedy for this mentioned in Fatwa No. 34504.