What is the ruling of Islamic law regarding a father who advised his nephew to abort a fetus resulting from an illegitimate relationship, and what is the expiation for that action, and is the father considered a killer of the fetus, given his knowledge of the act's unlawfulness and his inability to fast?

If a pregnant woman aborts her fetus after four months using medication or similar means, then blood money (diyah) is due. This diyah is a male or female slave, or five camels, to be paid to the fetus's heirs, excluding the perpetrator. Expiation (kaffarah (definition)) is also obligatory, which is freeing a slave; if no slave is available, then fasting for two consecutive months. If the abortion occurs before four months, it is forbidden, but without diyah or kaffarah. If someone orders an abortion and another person carries it out, then the liability falls upon the direct perpetrator, not the one who gave the order. Therefore, your father must repent, feel remorse, and increase his good deeds.