Saying Kaddish

Can a convert recite kaddish for a non Jewish family member who has passed away?
 
The 5th commandment, “Honour your mother and your father” is considered by the the rabbis to be a Torah precept that applies both to Jews and non Jews alike. The convert to Judaism is not obligated to recite kaddish for their non-Jewish parents. Their is general agreement among the rabbis that those who had converted were not prohibited from saying kaddish if they wish and, by so doing are fulfilling the requirement to give due honour to their parents.

Rambam, (Maimonides) with reinforcement from the Shulchan Aruch1, stated that one who has converted to Judaism must continue to respect his non-Jewish parents and must be recite kaddish for them.

There is rabbinic responsa which would suggest that this would also apply in regard to non-Jewish parents-in-law.

There are, of course, dissenting opinions also to be found. These would indicate that to do so would not in fact constitute a transgression.

(1)Shulchan Aruch Yoreh Deah 241:9