Because, at the end of the day, it's still family. From what I understand, Diana there were quite a few people with whom Diana was not on speaking terms when she died, which wasn't uncommon, as I think most of her relationships had ups and downs. Just because things were that way between them when Diana died (which was obviously sudden an unexpected) doesn't mean that she thought that relationship would've permanently been that way, or that she would've thought ties should've been severed between their children. Besides, Laura and William aren't responsible for any family spats. I think it's nice that a Spencer cousin was included this time (and a Middleton cousin, too), and I think what mattered most was who William thought was right as a godparent in the present day, rather than focusing on who was on good terms with whom in 1997.
