CNN — Kouri Richins, the Utah widow accused of killing her husband with a fentanyl overdose and then writing a children’s book about grief, is accused of witness tampering, according to court documents obtained by CNN. Richins faces murder and drug charges for allegedly poisoning Eric Richins with an overdose of fentanyl given to him in a drink the night he died. She has not yet entered a plea in the case and remains in custody, CNN previously reported. On September 14, authorities searched her jail cell and discovered a six-page handwritten letter to her mother, Lisa Darden, with instructions for her brother to “testify falsely,” according to…