Is a fetus resulting from an illegitimate relationship considered [the outcome of] zina (fornication/adultery) or zawaj (marriage) after a customary contract is completed? Is it permissible to abort it? And what is the ruling on assisting one's sister in doing so?

What this woman did is adultery (zina), and the 'urfi contract changes nothing, as it was concluded after the occurrence of adultery. It is also invalid Islamically (batil shar'an) because it was done without a guardian (wali). This contract does not make her the wife of this man, nor does it lighten her accountability before Allah. Abortion is a third crime, and she is not permitted to abort, even if some jurists (fuqaha') allowed it in its early stages, because that encourages immorality. The woman must offer sincere repentance, regret what she has done, and rectify her deeds.