The paperwork that’s at the link at the end of the article requires, as near as I can tell, 2 things:
1. that you’re sharing the same living space
2. that you sign a document saying you’re “committed” to each other.
I think most siblings are committed to each other. So if they room together, they should qualify. And if it doesn’t specify how strong the commitment must be, or the nature of the commitment, then I would think anyone you have a rental agreement with would also qualify.