He's a relative " by marriage, " we might say.

我们可能会说，他是“姻亲”。

The words for relatives by marriage are the same as with your immediate family, but with -in-law added.

姻亲用词同直系亲属，只是加了姻亲。

You could also have a relative who is not a blood relative, however – you could have someone who is a relative by marriage.

但是，您也可以有一个非血亲的亲戚——您可以有一个姻亲。