It depends on how you see relatives. The classification of plant families has been based on similarities in physical structures. Only pretty recently genetics came into play, screwing over large parts of past classifications.
The closer they are, the better they'll hold. But grafting herbs to woody plants seems like a long shot. There have been numerous attempts to graft cannabis onto hops because "they're the same family" but it never worked because even those two are too far apart.
It's hard to express in words 'how distant' relatives are. Even in numbers! We share a large percentage of DNA with chimpanzees, 97% or so, but grafting a chimpanzee liver into a human will most likely not work. In plants, sometimes we can graft a few branches of the phylogenetic tree, sometimes the entire tree, sometimes we can't even use different cultivars from the same branch.
I think your question is harder to answer than you'd suspect.