Today’s phrase is shirttail relative. I read it in a book, and decided to look it up.
Explanation: A shirttail relative is someone who is either a relative by marriage, distantly related (say, a third cousin), or a family friend who is an honourary “relative”.
Example: June invited everyone to the wedding – even the cousins of her aunts and uncles. She wanted to invite everyone to wedding, including her shirttail relatives.
Origin: This seems to have been around since the 1920s, and is mentioned in the the Dictionary of American Regional English as of 1927. It may have originated in the American south.