@omgfanimal If person A is talking to person B and doesn't know what the relationship is between B and C, he could say "I don't know what you guys' relation is." Or rephrased to be even more awkwardly, "I don't know what the relation of you guys is" (i.e. "X's relationship" is the same as "relationship of X"). I think "your relation" is both simpler and clearer.
"Your guys' relation" would be equivalent to "relation of your guys" which I don't think makes sense here.