A 26-year-old New Jersey woman was sentenced on Wednesday to 15 years in state prison for sexually assaulting a child in her home last year. Rachel Karngbaye, of Willingboro, must sever at least 85% of the term — or 12 years, nine months — before she is eligible for parole, the Burlington County Prosecutor’s Office said in a statement Wednesday. Karngbaye will serve her sentence in prison instead of a facility that treats sex offenders because an evaluation concluded that she is unwilling to undergo treatment, prosecutors said. She was convicted in September of sexual assault and endangering t…