I don't see a need to unfriend them, especially if you aren't ready to. Maybe just be a little more brief in your responses to them and eventually the relationship will dwindle. What you know about your deceased friend's relationships is between you and her and should stay that way. Eighteen years later her relatives wouldn't 'get it' anyway if you told them.

Maybe the real 'work' to be done is on your part ... the loss and the hurt you still feel in relation to her. That would benefit you more than anything you could do as far as her living relatives go. There may be a gift in this for you if you see it that way.