Colloquially, two phds from the same advisor are referred to as academic brothers/sisters, i.e. if i am a phd student with advisor X and someone else is a phd student with advisor X then we are academic siblings. In this framework, how would one describe the relationship of a postdoc to a phd, both having the same advisor?
I don't think there is any specific word for this.
In my experience, the "family" analogy (sibling, grandparent, uncle, etc) is only ever used for relationships defined in terms of the PhD advisor/student relationship. I've never heard anything similar used for advisor/postdoc or any other type of academic relationship.
I think you just have to spell it out. "He and I both worked with X; I was X's student and he was her postdoc."