How can I convince my grandma to start eating healthy?

Wow. That's a severe case. Like u said good thing she doesn't have diabetes. I don't know how you can convince her. She's already used to it. It's likely she won't change. But don't try to make her guilty about her habits. Umm. Maybe ask if you can cook for her? Not all Mexicans cook unhealthy. If she insist on having so much junk food like cakes and stuff, make her give it away to other people. Invite your friends over to eat the food lol. And Ask her to join you to go walking to the 99cents store. Stretch with her. Remind your abuelita that you love her, most of all, and keep lovin her even if she's like this until she dies.