What is the ruling on someone who swore, saying: "It is as forbidden as my mother for me to ever commit this sin," and then committed the sin a second time?

The statement "haram (definition) like my mother" has two scenarios: Either you intend to make your wife unlawful, which is a suspended ẓihār that takes effect when the condition is met and requires expiation (kaffārah); or you intend to make a specific action unlawful upon yourself, which is not ẓihār, but rather the prohibition of something that is already forbidden. In both cases, sincere repentance (tawbah (definition) nasūḥ) to Allah is obligatory, by abandoning the sin, regretting it, and resolving not to return to it.