What is the way to deal with a sister who is planning to undergo a sex change operation and is taking male hormones, despite it being proven that she is a complete female, and with the accompanying behaviors such as deviancy and separation from the family?

Gender reassignment is a great evil and is not permissible except out of a necessity recognized by Islamic law. If your sister undertakes it without justification, she will be sinful, as will everyone who assists her, based on the words of Allah Almighty: {And cooperate in righteousness and piety, but do not cooperate in sin and aggression}. You must advise her and warn her against this act, and seek psychological treatment for her if that is the nature of her feelings. If the matter necessitates seeking help from responsible authorities, such as the judiciary, then do so. Increase your supplications for her guidance, and if you have done everything in your power and she does not respond, then you are not to blame.