On May 11, 1999, Gary McCullough went missing after realizing his wife Sandy was having an affair with another man (and Gary had been her lover during her prior marriage, to Gary's brother, Albert). Sandy had six daughters who were fond of their stepfathers (the brothers McCullough) but one had a secret that she wouldn't reveal for years regarding Gary's disappearance. Then the daughter with the secret--Lena Chapin--disappeared too.


SOURCES:
1) https://www.radiotimes.com/tv/documentaries/true-crime/unsolved-mysteries-lena-chapin-what-happened/
2) https://unsolvedmysteries.fandom.com/wiki/Lena_Chapin
3) https://thelatch.com.au/lena-chapin-unsolved-mysteries/
4) https://www.phelpscountyfocus.com/article_43d12314-b585-11ea-8a73-2fd3ded7aa62.html
5) Netflix "Unsolved Mysteries" Volume 1, Episode 6