We often hear the term "cousin" used when describing a relationship to someone else. Most of the time it is not easily apparent how close this relationship might be. Wikipedia has a good explanation on defining this relationship. In simple terms, if you share common grand-parents you are first cousins, common great-grand-parents you are second cousins, etc.
This relationship has expanded from biological lineage to paternal lineage over the years when you use "love" as a criterion. It is a common contention that adoptive children are family. The is little or no distinction in the love relationship between a biological or paternal family member. Albeit appropriate to include non-biological relationships in this family tree, the genetic reality is that biological relationships accompanies inherited traits.