This is true, but I haven't really seen any instances where the servants are ordered to do anything like murder for any of the other guests.
Let's say that Kinzo said, "Shannon, if you kill these six people on the night of the conference, I will let you become the family head"... giving her the ability to do so.
Or (perhaps more likely), "Shannon, I will allow you to marry George if you kill these people."
So Natsuhi having Shannon serve tea to the guests does not at all conflict with her goal.
What I am saying is that while she is "furniture," she can have an agenda of her own.
So long as nothing the guests ask gets in her way.
More far-fetched theories, but I guess it's possible that Kinzo bribed someone (maybe not even his furniture) to kill his own children.
Those ungrateful children that only wanted his money.
Those ungrateful children that got in his way of resurrecting his lost love.
Maybe. Maybe not.