Some of today's most familiar words had different meanings previously. The change in meaning usually occurred in words referring to social relationships. For example, the word "cousin" often meant niece or nephew; and the title "Mrs." could show high social status, not necessarily marital status. There are a few other relationship terms that you should look out for:
The terms "niece" and "nephew" spring from Latin words which meant "granddaughter" and "grandson," so you may find them used in that context.
When we use the words "junior" and "senior," we normally think of a father and son relationship. However, in the past, these words were used much more liberally and could refer to an uncle and nephew, or even to two people with the same name who were unrelated.
The words "brother" and "sister" also were used in different ways. Members of the same church often referred to each other as brothers and sisters, and a married couple would refer to their brothers-in-law and sisters-in-law just as brothers and sisters.
The term "in-law" can also cause problems. In the past, "in-law" relationships could be either step relationships or the regular in-law relationship that we think of today.
Misunderstanding and misinterpreting these terms can really twist the branches of your family tree, so when you're reading older records it is important to be cautious. When it is possible, verify information with other records. This is the best way to make sure that you have the correct information. In addition, look at the rest of the language in the document. The more arcane terms and spellings you find, the more careful you should be.