Does the sin of using money acquired from an illicit relationship and spent on family fall away after it is returned once lawful money becomes available?

The burden of a forbidden relationship is not lifted by returning or disposing of the illicit money, but rather by sincere repentance. Illicit money that has been acquired should not be returned to its owner; instead, it should be spent on charitable causes and public welfare. However, if you do return it to its owner, there is no harm in that.