Do insulin injections invalidate the fast, and is a 74-year-old diabetic woman who is fasting obligated to fast?

Insulin injections do not break the fast because they are neither food nor drink, nor do they carry their meaning, and the default is that the fast remains valid. However, if your wife's mother is harmed by fasting, as informed by a trustworthy doctor, then she may break her fast and feed a poor person for each day, based on the Almighty's saying: "And for those who can withstand it [but with difficulty], a ransom [is due] the feeding of a poor person."