When a branch is broken from the family tree

Dylan and Shane have been friends for about two years. This last year they are always together. Shane is at my house on an almost daily basics. One day Dylan was at Shane's house, he called me and said that he thinks that they are related from my Mother's side Cummins. I told Dylan I know all the Cummins and either it is a long-long distance or another set of Cummins' family. A few weeks went by and Dylan was at Shane's house again and said that they were related. So, I talked to Shane's Mom and sure enough they are. Her Dad is my Uncle and we had the same Grandparents (Harry and Sylvia Cummins).
That is when I realized that my Uncle had two kids from his first marriage. I was very young way back then and just remember that they (the kids) stopped coming over to family get togethers.
I don't know what really happen, but it was not good. I am just glad that they will remain friends.
Picture above Shane and Dylan in the tree in the back yard.