A shirt tail relative is a distant connection, who may or may not be related by blood or marriage. It can be a second cousin or a family member connected by marriage and then divorce. Family is an expandable concept.
In Winnie the Pooh, Rabbit is searching for Small, one of his friends-and-relations. Rabbit has a lot of them. Rabbit organizes a search because he has not seen Small for a while. It takes a while before it becomes apparent that Small is not a rabbit….