How do you deal with it, Flaxwoven? I am not sure you can.
Read ToadintheHole's suggestions. Excellent advice. Problem is you are an anxious person, obviously not good at standing up to bullies - and this is what's happening here, let's not sugar coat it. Your DH, has clearly spent his life shouting "how high" when the older brother says "jump". A very hard thing to stop doing now, but for both your sakes and to keep the peace between you both, he needs to be the one to say, thanks but no thanks. AND STICK TO IT. It's a choice, not a command. If your DH doesn't stand up for what you both want, or he wavers, then you have no chance. For anyone to be thick-skinned and rude enough to think they can get you to drive when you don't want to, for a family member to remain blissfully ignorant of responsibility towards your diabetic DH says more about these two than you need to.
Be the two worms that turn. Think how important you are to each other, think how unhappy you feel, marching to someone else's drum at your time of life. Just do what you want, for heaven's sake. If others don't respond to your wishes and won't respect them, tough. That's their problem. This should have been addressed so many years ago - it never gets easier once people are used to having their own way.
You have to do this for yourselves. Honestly, self assertiveness sounds difficult. It's not - the first time you say No, mean no, and simply refuse to discuss it further, you'll never look back. The relief is incredible! It's much worse worrying about it - they're not, they're getting what they want any old time they want and it's time you gave their behaviour a huge shake-up. You can do it if you work together. Show your DH this thread. Good luck, you both deserve so much better.