Does my brother's rejection of a suitable suitor mean that God had predestined for me not to marry him, or did my destiny change due to his action? And is my brother compelled to do that or does he have a choice? And how does this reconcile with Divine Justice?

Everything in the universe is by Allah's divine decree and destiny. Humans are responsible for their actions and will be held accountable for them with reward or punishment, because Allah created them and granted them will and ability to choose their actions. Therefore, it is not permissible to use divine decree as an excuse for mistakes and sins. The questioner should differentiate between a servant's action and their accountability for it, and the action being created by Allah. If her brother has wronged her, she should report him to her father to discipline him, and the father must clarify the truth to the suitor. However, if there was a misunderstanding on her part, she should talk to her brother to clarify the matter. Forgiving her brother is better, provided that he is obligated not to repeat the act. She should also supplicate and be content with Allah's divine decree and destiny.