I am assuming this means a relationship that the writers didn’t write very well and so I would have to say Deanna and Mary Campbell. This mother and daughter relationship was really only shown through one episode 4.03 “In the Beginning” and merely mentioned in a few others, despite the fact that they are grandmother and mother to the two main characters of the show. I honestly don’t know anything about how these two women interact with each other because they are never even seen together without Samuel, their husband and father, also being in the scene as well. It’s almost the complete opposite of the Bechdel test.
Deanna, in particular, is simply a blank, obedient wife to Samuel, is totally underdeveloped and gets murdered after just a few, short scenes (which I talked about in more detail in Day 8). Mary also only ever mentions or shows affection towards her mother when she speaks about her parents collectively.
Additionally, I think this is very similar to what happened to Gwen Campbell, Sam and Dean’s hunter cousin, who was kept quietly in the background for a few episodes in Season 6 before being killed. Gwen never develops a good or close relationship with Sam or Dean and, like Deanna, I would have liked to find out more about her and, in doing so, more about the Campbell’s family tree.