I think your essay can contain both positive and negative elements of your experience having an autistic sibling. it does not have to be black and white and admitting negatives does not have to be complaining or whining, but can be matter of fact. Of course there are negatives. I don’t think your essay would be believable if only about the positives.
Another option is to write about it in the supplementary essay that asks what else you want them to know about you that is not in the rest of the application. But I think it is fine as the main essay too.
One of my kids works with children with autism and I have some idea of what your experience has been. I am sure your character will shine through, however you decide to write about it.