I saw an anime before. My brother pretends to be a brother. Is he a doctor?

According to your description, the animation I think of may be Reverse Supervision.

In this cartoon, the younger brother pretends to be his elder brother and becomes a doctor. My brother can't be a doctor for some reason, but he really wants to be a doctor. So he pretended to be his brother, went to the hospital and started his career as a doctor.

However, my brother's impersonation was soon exposed. The relationship between brother and brother has also become very tense. Finally, the younger brother finally understood his brother's difficulties and realized his mistakes. They made up again, and the younger brother finally found his own direction.

I hope this answer can help you! If you can provide more information, I will be more willing to help you!