What is the ruling on making up the days that a wife missed in previous Ramadans due to diabetes, especially since she believes that her fasting is for the sake of Allah according to her capacity?

If the wife is able to fast without harm, then she must fast. If she is unable to fast due to illness but can make up for it, even during short days, then she should break her fast and make up each day with a day. As for the days of menstruation, she must make them up. If she is suffering from a chronic illness that prevents her from fasting and causes her harm, and she is unable to make up for it even during short days, then she must feed a poor person for each day she misses. It is not permissible to rely on the word of a disbelieving doctor except out of necessity. In summary, if the wife is able to make up the fast without harm, then she must make up the twenty-four days, and expiation is not sufficient.