Under the Civil Code, an in-law refers to a blood relative of one's spouse.
The affinity relationship ends upon divorce.
姻族 refers to relatives connected through marriage (in-laws), the direct opposite of 血縁, which refers to relatives connected by blood.
From 姻 (marriage) + 族 (family, tribe). The exact historical derivation is uncertain, but the kanji compound directly conveys the meaning of 'marriage family'.