How does someone explain diabetes to another person who doesn't have it?

How old is BIL? If at an age where nearing end of life, then fighting this battle may not be worth the effort. If BIL is younger, then find a simple explanation of diabetes and how it impacts the body and give this to other relations.

Who or what is the "relation" that you are talking about?
A spouse? a Significant Other? a sister, cousin, child?
You really can not explain to someone that has no interest in doing what needs to be done. Even if they went to a class where this was all explained if they do not care to follow through there is not much you can do.
UNLESS..
If you think this person is purposely giving BIL an unhealthy diet.
If you think BIL is in danger of losing his life (a real possibility)
You could report the "abuse" to APS. This is a long stretch.
You could talk to a lawyer about trying to obtain Guardianship. (Do know this will cause a BIG rift in the family)
OR
You wait until there is a crisis and then try to step in. (if he recovers from said crisis and is not one that goes into a Diabetic Coma and dies in his sleep.

You might also discuss with the "relation" how much more difficult it will be to care for BIL when he is a double amputee and bedridden.

Who does BIL live with? Having had a diabetic husband and mother, I know learning the diet is a process. Until my husband was diagnosed with kidney damage, I did not treat the disease as a family one—which it is. And everyone benefits from following a diabetic diet—whole —not processed (like pizza rolls) —foods, whole grains, portion control with many small meals rather than a couple big ones, and yes, avoiding (or carefully limiting) the high carb items—fruits like citrus, grapes and raisins, watermelon, pineapple and “the whites”: white bread, pasta, rice, potatoes. Also high is corn (starches are sugars). The more fiber in something, the better.
It can be overwhelming at first. I recommend appointments—more than one—with a nutritionist. The doctor can write a referral. The nutritionist is the most important person on the diabetic’s care team in the beginning. It’s too much to try to learn on one’s own, even for the capable and motivated.
The family should attend the sessions. And it’s not just that eating the wrong foods will kill him. It’s the quality of life before he dies—with more energy and all his tors, both feet and legs, his eyesight. You are right to be concerned. Getting through to those responsible is his best hope. And making sure the house is filled with the right choices will help everyone not feel deprived.
Best of luck on this journey.

High blood sugar causes nerve damage and circulatory damage (damage to capillaries).

Nerve damage won't kill you but it can cause unremitting and often untreatable pain and, due to the risk of inadvertent injury because sensation has been lost, disability.

Circulatory damage leads to edema and contributes toward diabetic sores (often on the feet or legs) which may lead to amputation.

They may be thinking "Well, it's better he live now as he wishes" even if he drop dead a little sooner than he might have otherwise. But if they realize uncontrolled blood sugar causes irreparable damage that is far more likely to lead to serious quality-of-life issues rather than sudden death (years of increasing pain that often progresses to disability many years prior to eventual death) they might have a different attitude.

It's just like your teeth, except too much sugar weakens your whole body. The effect builds up slowly. Once your body loses the ability to turn sugar into energy, you can't get it back. (Just like teeth.)
*Every day*, you have to take medication (fillings). It can compromise his walking so you need a wheelchair. Or puts his eyesight at risk. If it gets really bad, you need a needle everyday (root canal).
They're all true.
Even if you have to repeat them, substituting sugar makes it easy to remember. Parents probably did it with their kids.
One meal at a time.
Good luck.

I had two friends with diabetes, one a juvenile diabetic, type 1 and one who had it later in life, type 2. Both had a leg amputated and both passed because of kidney failure, 63 and 69 years old. Diabetes effects every organ of the body. Because bad circulation in the legs happens, then they get sores that if they won't heal turn gangrene and thats where the amputations come in.

Its just not the carbs in those pizza rolls but the Salt totals 1850 mg. Average person should not use more than 2000 mg a day. Salt and insulin retain water. So the more sugar and carbs you eat, the more insulin you need the more water u retain. This relative is killing your BIL slowly. Your BIL should be eating good foods and keeping his weight down. Not that he can't have a treat every so often but better to control this with diet than to need insulin shots every day.

There is a difference between type 1 and type 2. I suggest you call your local Hospital and see if they have diabetic training available. Then u, BIL and relative attend it. You really need to understand diabetes to be able to control it. There are some fruits that should not be eaten because of the natural sugars. Carrots are a vegetable with natural sugar you should not eat. You need to know the signs of not enough insulin, too much insulin and low sugar levels. My girlfriend carried a small can of fruit juice with her all the time. Some diabetics carry jelly beans. Some diabetics get lethargic and need hospitalization.

This is a serious desease.

Ranting is a relief, this is a good place for it. Is your spouse involved in this situation, too? Can he be an ally? Maybe trying to set a limit - in our house, we serve the food, please do NOT bring any food to our house. They will push back at any kind of limit you set, but that might be one that could be enforceable. If they bring food, take it away and return it to them when the visit is over.
Trying to control blood sugar with diet alone is challenging even without this issue. Ask the doctor if there are medications he could take that will let you ease up a bit on this worry. Often diabetics who start with diet control progress to meds as they age, anyway.
Take care of yourself. You are a good person, to take on his care.

Talk to his doctor or look up a diabetic association online They will have information in easier to understand formats. Ask all of the family to review the information with you. Then, discuss together how everybody can help BIL stay on his diet. It may take a bit of conversation and brainstorming to come up with solutions to the "let him eat what he wants" to get same understanding of "what can we substitute for a high carb food" and "what can we do other than eat to show love".

If his short-term memory is gone you can't expect him to follow a diabetic diet or even understand one. "Counting" carbs will be an exercise in frustration for you and pretty much a wasted effort for all involved.

Are there low-carb foods he enjoys? If he likes "salty" things try giving him some dill pickles to snack on. There are some good sugar-free desserts (puddings, frozen yogurt) available. And low- sugar bottled nutritional drinks can be satisfying. Maybe some beef "jerky" could take the place of pizza rolls. Celery stuffed with soft cheese might be good for munching and help keep him feeling "full".

Ask the physician for a dietitian to spend some time with family to suggest appropriate foods he might enjoy. Then stock up on low- carb substitutes that he might be willing to eat. If he is elderly and forgetful, you may have to settle for keeping him full with low carb nutritious foods rather than continually trying to follow a traditional diabetic diet. If he has a visiting nurse or hospice they can check his blood sugar from time to time (or, if it worries you, you can, with a simple home glucometer--get a health professional or diabetic friend to show you how. Have a nurse or doctor explain to you how to interpret)

Maybe you could try explaining to them that his diet will kill him if this keeps up. Maybe they do not understand that.