I'd suggest structuring your keywords differently. You're mixing up a few definition concepts together, which is why the hierarchy is getting confused.

One of the items a person has is their identity. Another is their relationsihp with you. It'll be less confusing if you build those hierarchies separately:

People Names Fred Forsythe Sarah Jones Jonathan Appleby Relationship co-worker @ IBM co-worker @ Palm friend member of church scout troop Niece Grandfather

So when they're in a picture, you tag them with their name, and then tag them with the item(s) that define their relationship with you at the time the picture was taken. That may well change over time, especially with co-workers.

The definition of that person in that picture then is the set of keywords attached to the picture, rather than trying to create a single keyword that might be replicated to cover multiple situations into a single keyword. keyword sets become really powerful in defining a picture as you figure out how best to build the structures.

I've written in detail how I've built my keyword system, and a number of people have told me it's been quite helpful as they've been defining out their own.

http://www.chuqui.com/thoughts-on-lightroom-keywords/

(it'll be revised later this year, since I've started using synonyms a lot more extensively than I document in that piece. They're another way to handle this kind of situation in some of the situations, because you can attach those secondary characteristics to the main keyword and they get attached automatically. The downside is that if the relationships change and you edit the keyword (a co-worker changes jobs), that change is propogated to all photos, not just future ones, so the moment-in-time aspect of assigning keywords gets lost. but for keyword relationships that don't change (like, oh, "mom"), it can simplify assignment.