I am so sad to read what you have written, Bluefox and very sad for you - that you feel like you do.
I don't think you've said how long ago you lost your Father and it's likely that you're still grieving from the loss of both of your parents, especially with the additional struggle you have had for the longest of times with your brother.
I really can't see from anything you have written that your brother deserves a second thought from you - sister or not. I'm sorry if that sounds harsh but if you read what you have said from the point of you of describing someone's character or the way that they have behaved towards their own parents, I'm sure you would see that your brother is truly not a nice person. You've not been able to say a single nice or positive thing about him.
He was the golden child, you say. Maybe he was, maybe he was spoilt, maybe your parents let him know that he could do no wrong and he repaid them by doing everything he could to make their lives worse and being as selfish and heartless and thoughtless as he possibly could in their last years. You, on the other hand, can look at yourself in the mirror every day and know that you gave them all the care and concern and interest that you could. That you gave their lives in their final years your best shot, even though as you say, it put a strain on your own marriage.
From the few instances you have described, your brother was callous in his dealings with both your parents, has behaved appallingly and yet you spend so much time in agony thinking about him. Why? Why waste your precious time grieving for someone who doesn't deserve another thought and clearly thinks very little of you. He has chosen to estrange himself - so be it. It might help you to feel more positive and find the peace you deserve by speaking to a trained Counsellor - maybe even a bereavement counsellor - because you have suffered the loss of your entire family. It's time to try to turn your thoughts towards yourself, your Husband, your marriage, your plans for the future. Don't waste another moment - life is precious and wasting time on someone who has deliberately alienated himself from you is very unhealthy and you deserve so much better.
I send good wishes and really hope that you find the happiness and peace you crave.