So who is the rich half-brother going to be? Because there is no reason for that plot point if it's not one of the kids.
Doug does have a resemblance to them both.
While I do like the idea of that actually (it would be funny as hell imo and I do consider Doug my child now so lmao), I doubt the rich half-brother is in the mansion. Remember, Susana/Suzie was told she didn't NEED to be sent to the Shadow's House because she became rich and could just marry into another rich family. That implies that only children from rich families are exempt from the "privilege" of joining the shadows house. The working class folk of soot island send their children to elevate their status on the island (and I think its been implied there's financial benefits too), so having your kid chosen to become a part of the Shadow's House is very prestigious for those families in their communities. It's framed like your kid getting a scholarship to Harvard rather than the child human trafficking horror show that it actually is. That being said, since Susana/Suzie said that half-brother was born to be the heir of the family, I highly doubt they would send their only heir to the Shadow's House. They don't need the prestige the way working families on soot island do as that family is already rich. And a rich family with an heir would ask questions and look into the whereabouts of that child, whereas a working class family that does not have the means to do so wouldn't be able to find them even if they did start to have doubts about their child's actual whereabouts. Example: Mirror house family and their supporters have the capacity to ask questions bc theyre not brainwashed by the island's soot since they KNOW whats up, but do not have the financial resources to really do anything about getting back the children they sent to spy in the mansion.