They were cousins -- or relatives of some sort.
There's a theory that he actually wrote Mockingbird and she wrote In Cold Blood.
She helped him with the research and probably they read each other's books through before publication.
When I first moved to Garden City, KS I had 30 days of temporary housing. I was in no real hurry to find a place as I was making money on the temporary housing.
I discovered I was staying in the same Motel where Truman Capote and Harper Lee stayed while writing “In Cold Blood”.
I later saw where Capote said he was almost normal until he spent all that time in Western Kansas. I personally doubt it.