Does one say kaddish for a non-blood relative such as (chas v'shalom) a daughter-in-law? I know that a husband says kaddish when the parent of his wife passes away, but my understanding is that this is because women don't generally say kaddish, and he says it instead of his wife. With a daughter-in-law, though, the husband is still alive to say kaddish, so does his father say it or not?
The reason that I ask this question is because one might say that marriage adds someone to the family. I am basically asking whether that person is "part of the family" for the purposes of kaddish.
I know that one can say kaddish for anybody. I'm asking about halacha and/or common practice.