Bananas are pretty low on the Glycemic index and only have a glycemic index value of 51.
Bananas can help to raise your blood sugar levels if you have low blood sugar but it won't cause your blood sugar to spike instantly if you already have high blood sugar.
Bananas are a safe snack for diabetics to eat and are healthy as well but always check your blood sugar after eating Bananas to avoid causing your blood sugar to go up.