Keep in mind that relationships stated in letters, diaries, and other "unofficial" records may not be crystal clear or as specific as we would like. "Cousin" can cover a variety of relationships and a niece or nephew may be a niece or nephew by birth or by marriage. A newspaper account may refer to someone as "Grandma" simply because of her age.
Legal documents are usually more specific,but even they can be in error. Estate and inheritance papers are more likely to state relationships correctly and specifically.