A Family (or often more formally referred to as a Nuclear Family is defined, in a historical, legal, religious and tradition (and thus in genealogy which tries to capture the ancestry) by the marriage of a husband and wife who then procreate to create offspring that they care for and raise to maturity. Now, before we get all the hate mail and start major flame wars, we again simply document how it has been treated for centuries and thus what a genealogist is trying to understand and document.

A Household is simply a collection of people living together.

Siblings in a household may or may not be formally recognized as related; independent of whether they understand or know this. Genealogical tools tend to only capture nuclear families and not households. (Formal) adoption is usually the only way to include a child in a family that was not born into it by the parents that define the family. See the siblings page for more information on the various types and flavors of children in a family.

More lenient genealogical tools will allow for the documenting of non-traditional families as if they were a Nuclear Family. This even if the laws, customs and legal rights of the time did not recognize it. Clearly, same-sex parents is a classic example as this has existed, albeit if under the radar and not widespread, for generations.

With the recent advent of DNA testing coming to the general population and being heavily used for genealogy, it has become even more important to clearly document and distinguish the various flavors of family. Whether known to exist or not. When DNA points to a biological parentage different than has been documented and accepted in the family, genealogists refer to this as a Non-Parental Event (or NPE for short). This definition occurs while the work to explain the discrepancy is resolved and the understanding of what is the new, likely order is accepted.