You wrote "My dad had a sister who married a Welch and a brother, John."
My father, John Joseph Scully, was born in Boston in 1908. His father, my namesake, was Michael. John's sister, Margaret (my Aunt Peg), married Warren Welch. They had three children who grew up in the Boston area. My dad also had a brother, Francis or Thomas I believe, who died either before I was born or when I was a small child. He had married and his family settled elsewhere in New England.
My father moved to NYC, where I was born. He died 40 years ago today, when I was still a kid, which is what prompted the idle web searching that led me to your message. I know nothing of the whereabouts of my cousins. These would be my Aunt Peg and Uncle Warren's kidsand those of my father's brother.
I can't answer your specific question, but are you one of my cousins?