Encyclopedia of Human Relationships
Sage.
Available online. (W&L subscription)
Three-volume set (2009) contains entries up to 3,000 words in length offering "an interdisciplinary view of all types of human associations --- friends, lovers, spouses, roommates, coworkers, teammates, parents and children, cousins, siblings, acquaintances, neighbors, business associates," etc.
Links will be maintained in several locations in the library Web site, including here.