What harms can corset waist training do to my ribs? (bones)?

My first instinct would be to tell you not to do it, to enjoy your body the way it is. But moving past that, I think breaking the bones would only happen if you do it too quickly. Be very gradual about it, and make sure you have enough calcium and vitamin D in your diet. You will be changing the way your bones are shaped, so you need to make sure they are strong enough to handle it.