Is it ok to sister a joist to burnt joist?
As a home inspector, I can tell you that sheer existence of those damaged joists are going to be a RED FLAG at any time you decide to sell and move on. If they are badly burned, they are not structurally sound and should be removed. Install a few temp supports on ends of adjacent joists, then remove and replace them one or two at a time. Use proper joist hangers, proper intervals and upgrade the size if current code requires. I little more work now will pay off big for you later. The cost will not be much different, just a bit more labor.