The terms for that difference in relationship reflect a difference in behavior. For example, as a child you probably expected gifts from your grandparents, but not from your great-uncle or aunt. Notice also that there is a difference in "terms of reference" ("father, other"), and "terms of address" - "dad", "pop", "mommy", "mom", etc. The difference in "terms of reference" or "terms of address" could be confusing in study.