How can one deal with feelings of not accepting a twin brother after the father's death, especially with the feeling that he is trying to replace him in family matters, and the feeling of insecurity, deteriorating self-confidence, loneliness, and academic decline?

We advise you to be patient and seek help from Allah Almighty, and not to feel weak or in need of any created being, and to increase in the remembrance of Allah. You should also discuss family matters with your mother and siblings, accept your brother, and strive to occupy yourself and your time with what is beneficial, such as your medical studies.