Siblings or dating
There are two types of twins: identical and fraternal.
Identical twins have exactly the same genes; fraternal twins are no more similar than regular siblings.
They gathered subjects with a mean age of 59, who included 99 pairs of identical twins, and 229 pairs of fraternal twins who were all reared apart.
They also gathered twins who were reared together: 160 pairs of identical twins, and 212 pairs of fraternal twins.
In law, the term consanguine is used in place of agnate).
They share only one parent instead of two as full siblings do and are on average 25% related.
One's sibling-in-law is the sibling of one's spouse or the spouse of one's sibling.
They are the children of the godfather or godmother or the godchildren of the father or mother. In cultures with milk kinship, a milk sibling is a person who is not one's biological sibling but was nursed by the same woman as oneself. Cross-siblings are individuals who share one or more half-siblings; if one person has at least one maternal half-sibling and at least one paternal half-sibling, the maternal and paternal half-siblings are cross-siblings to each other.
This study also suggests that heritability is substantial, but not as substantial as for younger subjects; it has less significance later on in life. They may share the same mother but different fathers (in which case they are known as uterine siblings or maternal half-brothers/half-sisters), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-brothers/half-sisters.Three-quarter siblings are likely to share more genes than half siblings, but fewer than full siblings.In this case the unshared parents are full siblings.Furthermore, the three-quarter siblings are also first cousins.In the case where the unshared parents are identical twins, the children share as much genetic material as full siblings do.
Stepsiblings are not cross-siblings unless their married parents have a child together.