How related are two relatives?
Python 3, 259 247 226 bytes



lambda l,a,b:sum(2**-len({*m}^{*n})for v in{*sum(l,())}for m in p(v,a,l)for n in p(v,b,l)if not{*zip(m,m[1:])}&amp;{*zip(n,n[1:])})
p=lambda i,j,l:sum([[[r+[i]for r in p(y,j,l)],[[y,x]]][j==y]for x,y in l if i==x],[])+[[i]]*(i==j)


Try it online!

This can probably be golfed down quite a bit, but for now I'm just happy it works.