Truth?

Riddle: 3 doctors say that their brother is Mark. Mark says he doesn't have any brothers. How is this possible?

Answer: SISTERS! The most annoying beings on the planet. Or that's at least what my brother say about me and my sister...

Source: https://www.riddles.com/4162

3 doctors say that their brother is Mark. Mark says he doesn't have any brothers. How is this possible? — Truth? by GoneGirl v1.

Riddle: 3 doctors say that their brother is Mark. Mark says he doesn't have any brothers. How is this possible? Answer: SISTERS! The most annoying beings on the planet. Or that's at least what my brother say about me and my sister... — by GoneGirl v2.