From: Rabbi Moshe Sofer
To: Rabbi Yaakov Ber
An orphan raised by a deceased man wishes to recite Kaddish for his adoptive parent, but the biological children refuse to share their allotment. The Chasam Sofer rules that the mourners are correct, as the orphan has no formal obligation of kibud av toward the one who raised him.
Source: Shu"t Chasam Sofer OC §164