Answer:
President James Buchanan
Step-by-step explanation:
The US President who had his sister initially serve as the first lady was President James Buchanan. Buchanan's niece, Harriet Lane, acted as his hostess and assumed the duties of the first lady during his presidency from 1857 to 1861.
At the time, Buchanan was a bachelor, and his sister, Jane Buchanan, had passed away prior to his presidency. As a result, Harriet Lane, his niece, fulfilled the role traditionally carried out by the first lady.