What is the expiation for someone who broke their fast in Ramadan of the year 1431 AH due to illness and forgot about it, with their brother reminding them of it?

If someone doubts whether they missed a day of Ramadan, they are not required to make it up, based on the principle of the presumption of innocence. However, if your trustworthy brother informs you that you broke your fast on a certain day, and you predominantly believe him, then you must make up that day, unless you are certain you fasted it. In the event that a make-up fast is obligatory, only the make-up is required, and there is no expiation for forgetting.