What is the advice for the brother of a girl whose treatment of his sister and her fiancé has changed for no reason, and he has come to hate them, even though they have committed no sin in Islamic law?

Your question is reprehensible. Recall your forbidden actions and do not ask about repentance from them, then ask about another matter that is not reprehensible.

If your brother's abandonment of you two is due to your forbidden relationship, then his abandonment is permissible.

Our advice to you is to return to your senses and repent to Allah for the forbidden acts you have committed, and to cut off your relationship with that woman. What you mentioned about seclusion (khalwa) and embraces is forbidden by both Islamic law and human nature.