Solange Knowles reportedly attacked Jay Z after she "overreacted" to something he said.
The 27-year-old singer "flipped out" at the rapper, 44, in an elevator at the Met Gala after-party in New York last week because she was offended by a comment he made.
A source told Us Magazine: "Solange flipped out over something Jay had said, that she took the wrong way. She took it too hard or too far.
"Not sure what it was, but she greatly overreacted."
Solange was caught on surveillance - obtained by TMZ.com - kicking and clawing her brother-in-law as they left the bash held in the Boom Boom Room at The Standard Hotel in New York City, last Monday.
Jay didn't appear to retaliate as the brunette beauty screamed in his face, but he did attempt to hold her hands down as their bodyguard struggled to hold her back.
The bodyguard can even be seen pushing the emergency stop button repeatedly as the elevator reached the 12th floor in a bid to keep the violent altercation private.
Beyoncé appeared to stay calm and collected throughout the scuffle and watched on as her sister violently attacked her husband.
However, at one point, the XO hitmaker - who has two-year-old daughter Blue Ivy with Jay - intervened and said something to Solange after she began punching him again.
Solange then stormed out of the elevator with a disgruntled expression plastered across her face as Beyoncé, 32, and Jay followed shortly behind.
The siblings were then seen getting into a car together, while the Holy Grail hitmaker opted to travel alone.
The Standard Hotel in New York City says it is investigating the leak of the video.
Asked about the video, the hotel issued a statement saying it is "shocked and disappointed that there was a clear breach of our security system".
TMZ posted the video and said it was shot last week inside a Standard Hotel elevator.
The hotel says the leaked video violates the confidentiality it provides customers. It says it "will discipline and prosecute the individuals involved to our fullest capacity".
Representatives for Beyonce, Jay Z and Solange didn't immediately comment.
- Bang! Showbiz with AP