I read that twice and I'm still not sure I understand.My father and mother had a smililar situation. My dad's family had eleven children. He was the youngest. By the time he was born some of his older brothers and sisters were already married with kids. He was an uncle the minute he was born. An older sister married a man named Joe. When dad grew up he married Joe's niece. Dad's sister became his aunt by marriage and my mother's uncle became her brother-in-law by marriage. His wife, whom she had long known as an aunt, became her sister-in-law. My aunt is also my great aunt because she married my grandmother's brother. Her husband was both my uncle by marriage (married to aunt) and my great uncle by blood ( brother of my grandmother). When telling this story I take great pains to explain that my parents were NOT related!
It sounds like something I found in my family tree. My maternal great grandparents were 2nd cousins because his grandmother and her grandfather were brother and sister. But wait, it gets worse. The above mentioned brother and sister who were my great grandparent's grandparents, well their parents were uncle and niece. It wasn't an old uncle and a young niece. They were about the same age and he was the youngest brother of her mother. This is very far back in the family tree as my great grandparents were born in the late 1800's and both died before I was born.