I have a water blister on my forefinger from a burn. should i break the blister. i have diabetes?

Don't break blister: It is always better to leave a blister intact. Once you open it then there is a way for infection to get into the damaged skin underneath. This is especially important in diabetics. I suggest leaving the blister intact and putting a bandage over it. In a few days it will probably open on its own but by that time then skin underneath will have had some time to heal up.