Eric Peeler, 25, of Post Road East, was accused by police of hitting the sibling while he or she was asleep.
The attack, police said, appeared to be unprovoked.
Peeler has been charged with third-degree assault and risk of injury to a child, police said.
He was held on a $10,000 bond.
Peeler had a scheduled court date Monday, May 16, but it was not known what the outcome of that was or whether he made bail.
It was also not known if the sibling was badly injured in the alleged attack.
Click here to follow Daily Voice Westport and receive free news updates.