I remember reading somewhere that he did those studies because a bunch of his kids died young and he wondered if the inbreeding had something to do with it.
Nah. Most of their children were entirely healthy and happy and grew to become successful adults. 2 died of ill health during infancy, which in the 1840s wasn't uncommon. And one of them was because he had Down's Syndrome due to the mother being 48 at the time of his birth.
It was the death of his second daughter Annie that affected him and he blamed himself for her death of from tuberculosis, wrongly believing it was a family trip to the beach which caused her to catch a cold.
Prior to the 1900s and the invention of the automobile, it was quite common. Biologically there is not a major concern if you share grandparents but not parents. The issues only start to pop up with repeated mixings when no new DNA is inserted into the mix as ressive traits will be inherited
Einstein's second wife was his first cousin on his mother's side and second cousin on his father's side. That is probably not so good, although they didn't have any kids. Einstein's children were all from his first marriage.
Where do you think he got his evidences from?