A 9-year-old autistic boy will not face charges after he allegedly slashed his baby sister in his family’s Brooklyn apartment, police said Thursday.

The boy’s family told The Post that the boy cut his 14-month-old sister in their Bedford-Stuyvesant apartment on Tuesday while the children were home with their great-grandmother.

The woman, Elaine Thomas, said the toddler was expected to return home from Bellevue Hospital on Thursday.

The family declined to comment Thursday or discuss the case, and it was unknown if the baby has returned home

Thomas said Wednesday the boy admitted he cut his sister after she walked in and saw the girl “covered in blood.”

Additional reporting by Kevin Sheehan