The stepfather wasn't trying to be the kid's father. He was letting the kid know what he saw and wanted the kid to give his mother a chance, as well as his new living arrangement a chance. He wasn't trying to tell the kid what to do. He was telling him what was on his mind. He was trying to reach out which is more than the kid had done.
You need to stop complaining about how unruly and disobedient his dog is and tell him why his dog is so hard to care for. If he won't train his dog properly, maybe when he sticks you with his dog, take him to a trainer and get the dog trained to obey commands and to stop pulling on a leash and learn not to whine when his human leaves.
The answer was staring you in the face. Ask your uncle that you were close to. Who cares if he's not related by blood. He was like a second dad to you. He's the obvious choice. If your relatives cannot get along with him for one day, they don't need to be at your wedding.