How can it be said that Al-Qasim was the eldest son of the Prophet (peace be upon him) by Khadijah, considering that she bore all his children except Ibrahim, while Khadijah was his first wife and he did not marry another during her lifetime?

Ibn Hisham said that Khadijah, may Allah be pleased with her, bore all of the Prophet's children, peace and blessings be upon him, except for Ibrahim. They are: Al-Qasim (by whom he was known by his kunya), Al-Tahir, Al-Tayyib, Zaynab, Ruqayyah, Umm Kulthum, and Fatimah. This wording does not mean that Al-Qasim is Ibrahim, but rather it is an explanation of the children whom Khadijah bore.