The concepts of relatedness - Hamilton's rule and kin selection
Is that really trying to imply that there are significantly different definitions?  It seems to me the intent was just to put it in terms of an equation with more useful numbers.  This terrifying paper from 1964 gives some of the follow-up works, in particular works from Cockerham in 1954 and a number of papers by Kempthorne, in particular this from 1955 and this from 1963.  There is also Malecot's method of coancestry from 1948. (WorldCat.org link)  The idea was mainly to look relatedness outside of the specific limitations present in Wright's work.  Some of the math in those are more accessible than others...