SIBLINGS: Siblings are the children of the same parents. It is a convenient term meaning "brothers and sisters". If you have two brothers and one sister, you have three siblings.
SPOUSE: Another convenient term meaning "husband or wife".
HALF-: If your remarried parent and your step-parent have children, those children are your half brothers/sisters. Because you have one parent in common, you are partly (half) related "by blood".
IN-LAWS: If your brother or sister gets married, his/her new spouse becomes your sister-in-law or brother-in-law, but the family of your new sister(brother)-in-law does not become related to you, only in-laws to your newly married sibling. When you marry, however, you do acquire a whole family of in-laws of your own. Your new mate’s family now become your father-in-law, mother-in-law, sister- or brother-in-law. In-laws, like step-relatives, are not related "by blood" but by "extended family ties". Note that the term sister- or brother-in-law is used both for your spouse’s sibling and for your sibling’s spouse.
GRANDNIECE/NEPHEW: The children of your siblings are your nieces and nephews. The children of your nieces/nephews (your siblings’ grandchildren) are your grandnieces/nephews, and their children are your great-grandnieces/nephews. Grand-nieces/nephews are also sometimes called great-nieces/nephews, and great-grandnieces/nephews are also called great-great-nieces/nephews.
GRANDAUNT/UNCLE: The siblings of your parents are your aunts/uncles. The siblings of your grandparents were originally termed grandaunts/uncles and the siblings of your great-grand parents were great-grandaunts/uncles. But over the years those terms have gradually been replaced by the less descriptive great-aunt/uncle for grandaunt/uncle and great-great-aunt/uncle for great-grandaunt/uncle. Because it is more logical, many modern genealogists still prefer to use the older terms. Either is correct.
NAMESAKES: If your name is John Henry Doe and you name your son John Henry Doe, you then become Senior (Sr.) and your son Junior (Jr.), not the second (II). If your son John Henry Doe, Jr. names his son (your grandson) the same, his son then becomes ‘the third"’, i.e.., John Henry Doe III. However, if you name your son Richard Henry or John Harold, anything but John Henry Doe, but he still names his son (your grandson) after you, your grandson then becomes John Henry Doe II. Likewise if your brother Edward Charles Doe names his son after you, that child (your nephew) would also be John Henry Doe II. A ‘junior" always has the same name as his father whereas "the second" is not named for his father but does have the same name as an older relative (grandfather, uncle, cousin, etc.). The ‘third" is the third descendant in a family with the same name in either direct or indirect line. In everyday practice, the Sr., Jr., III are often only used when all parties are living but genealogically it is important to maintain the correct title to prevent confusion.
GENEALOGY versus FAMILY HISTORY: Although currently the two terms are frequently used interchangeably, there is a difference between a genealogy and a family history. A genealogy starts with one ancestor, most often the original immigrant to the United States, and traces all his descendants to the present time. If that ancestor arrived on these shores in colonial times, you can imagine the hundreds and hundreds of descendants he now probably has and what a mammoth task it must be to find even half of them! A family history starts with yourself (or your children) and moves back through your two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents, thirty two great-grandparents, etc., spreading out fan-like to discover all the people from whom you are descended, not just names and dates but preferably information about their lives and backgrounds. Most genealogy courses offered today concentrate mostly on techniques for a family history rather than a genealogy. However, many of the methods can be used for both.
MATERNAL/PATERNAL RELATIVES: In our English language, we do not differentiate between the maternal (mother’s) and paternal (father’s) sides of the family as is done in some languages. While this is handy for common speech, it can be frustrating to genealogist or family history researcher. For example, if you found an old family letter which wrote of "my grandmother returning to her home in Smithtown", you could not be sure whether it referred to the writer’s maternal or paternal grandmother. If the letter had been written in--say Swedish-- the word for "grandmother" would have been either "mormor" (mother’s mother) or "farmor" (father’s mother) and it would have been clear.