What is the ruling on money acquired from an unlawful relationship, and must it be returned to its owners after repentance, or is it permissible to spend it on one's parents?

The money taken as a pure gift is permissible for you, and it is not obligatory to return it. The money taken in exchange for a forbidden relationship is unlawful, and you must spend it on the welfare of Muslims and charitable causes. As for what was taken as a loan, it must be returned to its owner. If it is impossible to reach him, then give it in charity on his behalf. It is permissible for you to benefit from it if you are poor.