You are right to be concerned. But your brother-in-law is in denial. He feels fine because his body has gotten used to operating with high blood glucose levels. But that high level is destroying the small veins in his kidneys (which leads to failure) and in his overall circulatory system which can cause heart attacks and strokes. He will develop diabetic neuropathy (nerve loss in his hands and feet) and a condition called autonomic neuropathy where his brain is affected and does not register when his blood sugar goes too low, which can lead to a coma. He faces amputation of his limbs and blindness. Maybe you can put the fear of God into him with our posted replies. I don't know; some people are just unreachable. I hope he is not one of them. Good luck to you.
Waverider got it rite he IS going to have to pay the piper
Sorry to see your bro-inlaw is in denial about his diabetes. By ignoring blood sugar control his levels will continue to rise. This leads to serious health complications down the road. If he's looking forward to going blind, or losing a foot or both feet, or having bacterial skin infections, or kidney failure, or a stroke he can continue his current lifestyle. Hopefully you will find the trigger which changes his mind set so that he will start to take better care of his health. Good luck -