The prefixes great-, grand- and great-grand, and the suffix -in-law can be used to express a number of family relationships. Some are very common and others less so. Here is a list of the possible relationships:
great-aunt, great-uncle, great-nephew, great-niece
grand-aunt, grand-uncle, grand-daughter, grand-son, grand-mother, grand-father, grand-nephew, grand-niece
great-grand-mother, great-grand-mother, great-grand-son, great-grand-son
brother-in-law, sister-in-law, father-in-law, mother-in-law, son-in-law, sister-in-law
A great-aunt is the sister of a grandparent. A great-niece is the daughter of a nephew or niece.
A grand-aunt is the aunt of one's parent. A grand-nephew is the son of a nephew or niece. Thus the words great-niece and grand-niece have the same meaning.
The suffix -in-law is used to family members who are not related by blood but rather by marriage. The four affixes great, grand, great-grand and -in-law can be used to signify a large number of family relationships. In certain cases, grand- and great- have the same meaning.