I've tried finding the realtionship of Alfred Feinberg to my husband's family. Alfred was a well known artist and medical illustrator. We aren't sure if he is a great uncle or a cousin. I have contacted the Guggenheim Museum in NY City and they directed me to an article about him in the NY Times. I ddin't find any info that would show the link to our family. My mother-in-law was a Feinberg by birth. Her father was Louis Feinberg.
Olive Tree Answer: Hello Marcia. You didn't give me any dates or locations for your family but I can advise you re steps you should take.
First, you must research each family - that of your mother-in-law, and that of Alfred Feinberg. Search all siblings and keep going backwards. With each new generation you find, be sure you find out complete family groups (siblings etc).
I don't know when Alfred lived so perhaps you will have to come down his line to meet up with your mother-in-law's? Or perhaps you will have to further back to find a common ancestor. But there is no shortcut other than researching each family completely in order to see if there is a connection, and how they connect.