My friend and her brother have bern doing this for closing in on 10 years. They become each other's OP, without any romantic entanglements. Just mistreat each other terribly. My friend keeps expecting something different and I keep asking "Why would you? You were not good siblings when you were younger, what has changed? " The answer is nothing, they still haven't worked out their issues. My friends brother does none of the standard day to day things normal people do, like paying their bills, then acts put upon when the power is turned off. If my friend nags him to pay the bills, he resents her for the reminder that he doesnt pay them. She's like a mother, which makes sense since their mother was not responsible and their father was (imo) a jerk who went around insulting everyone to make himself feel better.
It's kind of fascinating to watch, having known the family dynamics.