If anyone gets possession of a relative who is within the prohibited degrees (of marriage), that person becomes free
Authentic hadithعن سمرة قال: قال رسول الله صلى الله عليه وسلم : «من مَلَكَ ذا رَحِمٍ مَحْرَمٍ فهو حُرٌّ».
Samurah reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "If anyone gets possession of a relative who is within the prohibited degrees, that person becomes free."
Explanation
This Hadīth illustrates that anyone who owns someone who is a close relative to him in such a way that it entails prohibition of marriage, then that person becomes free once he is owned. For example, if someone comes to own his father, mother, sister, aunt, etc. through purchase, gift, booty, or inheritance, they are to be set free.
Source · Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad
From the Hadith Encyclopedia (hadeethenc.com).