Siblings have variations in meaning in the legal, biological and genealogy context. But generally, there are either Full, Half or Step siblings.

Full Siblings share the same two biological parents and thus share, on average, about 50% of their DNA (as measured in the Full-Identical comparison; or 37.5 to 42% as measured by Half Identical tools). Full siblings, by definition, are in the same nuclear family defined by the biological parents; whether in a formal union of marriage or not.

Half Siblings only share a single parent. They thus tend to only share 25% of their measured DNA — either by Full-Identical or Half Identical measures as they have no full-identical regions.

Step siblings share no biological parents and thus should, in general, share no DNA.

Some in Genetic Genealogy have been seen to use a term 3/4 Sibling. This is simply meant to imply half-siblings who share one parent and the other parents are full siblings. For example, a widowed mother remarries her husbands brother and has another child. Those children would be 3/4 Siblings. Seems this occurs often enough to give it a term.

All these terms are really focused on indicating changes in the amount of shared DNA from the expected or traditional Full.

Step relations often happen after a parent with children becomes widowed, and remarries someone who also has their own children from another relationship. These children get pulled together in the same household but do not share any biological parents and thus are Step to each other. If the new couple have more children, the household now may have full, half and step siblings in the same blended household. If a new parent adopts the children they did not biologically generate, then the child becomes part of the new nuclear family from a legal and genealogy standpoint but are still distinguished as step from a biological standpoint. This because laws and religious traditions related to relations between kinship apply differently to biological related people than not.

Young CHILDREN in the same household often simply consider themselves Siblings (or full siblings) independent of any biological or legal measure that says they are not part of the nuclear family. Genealogists do not consider non-full-siblings part of the same nuclear family when recreating the ancestry. Legal Adoption is the only “recognized” way for a child to become part of a nuclear family as much as a full-sibling would be. This is a shame because households of young children with other than full siblings really behave as and believe they are all in a nuclear family; independent of any biological or legal action that may or may not have occurred. Legal action of adoption was rare before the last 100 years. So you may see genealogists add adoption to a child (and notate it as invented) simply to force the genealogy tools to include the young child in a new nuclear family. This is to reflect what the child/person considers their siblings through their whole life.

The terms Full, Half and Step do not generally apply outside siblings and the nuclear family but some may extend it to first cousins. For example, half first cousin to mean first cousins who are children from half siblings.

For the closeness count (i.e. consanguinity), a half relation adds one more “step” in the genetic distance. Say going from 4 to 5. Step are not related biologically and so have an “infinite” genetic distance.

From a computer programming perspective, it is very useful to think of siblings as 0th cousins. Then algorithms and processing are simple when determining relationships. Computer Scientists always like to start counting at zero :-)

When a half-sibling is not previously documented in records or formalized, genealogists like to say a Non-Paternal Event (or NPE) occurred. This is simply a catch all to explain that the DNA matching will not reflect the previously documented nuclear family. Genealogical tools have a way to distinguish adopted from biological parents, but no formal ability to specify what was the real household (or even nuclear family as the child perceived it. Recent formalization of things such as surrogacy, sperm and egg donors, and similar items only serve to make the whole process that much more complicated. Even experimentation occurring recently is causing DNA to come from three parents instead of two. So these traditional terms and meanings are bound to evolve in the legal, medical and general communities overall.