From the article SRS mentioned:

"Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man presents itself as the first genuine psychological portrait of Trump by a person who knows him well. It is written by the president’s niece, Mary L. Trump, who not only has the firsthand experience with the president and his family that his would-be shrinks lack, but is also herself a clinical psychologist by training."

Mary Trump is a qualified clinical psychologist therefore she is capable and qualified to provide a psychological diagnosis.