A 21-year-old Bronx man was being questioned last night in the strangling of his younger brother, police said.

Alex Williams, 18, was found unconscious and bleeding from his nose and mouth at 1:50 p.m. Thursday in the family’s Bronx Boulevard apartment by his father.

The Medical Examiner’s Office said the teen had been strangled.

“He was a nice person, cool, respectful,” said former girlfriend Bianca Scott, 17.

Williams’ brother, Gavin, was taken in for questioning, but no charges were filed by early today.

“He and his brother were completely at odds,” one friend of Alex told The Post. “They were like cats and dogs.”