So you have that. Back when all families were large, it was typical for the eldest to play the role of a parent. I know my great-grandmother did that, being the eldest of perhaps 7 or more kids, born to a teenage mother as typical back then. For modern times, 4 is a lot and you were served that lot in life. With autism, even mild, it is understandable that your parents are protective of the boys, but still it is unfair to you to be treated as the only one responsible. And your sister clearly does not have autism.

Have your parents ever told you that you would need to bear the responsibility for the brothers when they are gone? Sounds like no, it was with the sister that this idea of you becoming the parent when they die was uttered. Or is the autism so mild that they can function fully independently?