Welcome Rainbow. Diabetes is a serious condition and your dad's numbers are not good.
The reason you dad may not be taking more positive action to manage his condition may be because of ignorance of what he could do to help himself or perhaps he believes that there is nothing he can do to help himself.
Diabetes is a long term chronic condition with very negative publicity. Only lazy greedy fat people who don't take care of themselves get T2. They have only themselves to blame, or at least that is what you'd think if you read the media. And the picture given even if you do change behaviour is very negative. Hardly motivating, and a recipe for comfort eating, I'd say. I perfer to believe that the weight gain and hunger are symptoms of rather than the cause of T2 diabetes - there is evidence to support this viewpoint and I find it much easier to take responsibility for bring my numbers down if I'm not racked with guilt and self hate.
Also, the NHS has very low expectations of what good management is and also about a diabetic's ability to change his/her behaviour. I'm told my last HbA1c of 7% is good enough control for a T2, yet these levels are still toxic to my blood vessels, my nerves, my eyes and my poor beta cells that produce and manage my insulin production. So I'm aiming to lower that considerably by adapting my diet. This can be done.
However, it is the diabetic that has to get with the program, not the family and friends. Although supportive family and friends make a huge difference to how succesful someone is in the changes they make. It is pretty difficult to be told "you can't have that" when others are tucking in. A lot of the advice diabetics are given from well-meaning family and friends can sound a lot like nagging.
You don't say if you live with your father or what the rest of the family's attitude is. If you do live with him, you can clean up the larder to provide healthier choices so that everyone is making healthier choices. But unless you supervise him 24/7 he will find opportunities to buy the high carb rubbish that many people think passes as a healthy choice for a diabetic.
The best advice I can give you is to be selfish. You may or may not have inherited the diabetic genes - T2 is inherited - you just don't know. I have T2 because I have diabetic genes. You 9and any brothers and sisters you have) may want to consider what your options are for minimising the risk you will develop T2 at some point in the future by making some dietary choices now that will significantly improve your outlook.
The best advice I can give you is to be seen to be addressing your risk and dealing with things from your perspective. Have some handy reference materials to hand - I'd recommend "Bloodsugar101: What they don't tell you about diabetes" by Jenny Rhul (even if it is written in American) and Charles Clark "Diabetes Revolution: A Groundbreaking Guide to Reducing Your Insulin Dependency" which is british.
Your dad has the right to make bad decisons about his health and it is really hard to stand and watch this. But that is what you have to do because you can't be with him 24/7. Your own health is another matter and if you do that publically, well some of it might rub off, but trying to manage it for someone just creates difficult situations, in my opinion. I wish I'd understood exactly what insulin resistance meant to my body 3 years ago and what I could have done to improve things.
Take a look around here, especially at the food forums. There is lots of support and good advice.