Yes, Brandon is a blood-related brother in the 1st and current world ‘Obsession’.
There are content warnings on the Warning Tab for each route, so please review them all! If incest is a no-thanks for you, do not play Brandon’s route.
Other routes have references to Brandon’s past abuse of his sister, and there may be a rare scene where he flirts with her as well. These are brief scenes with limited detail that exist for story advancement.
The next world “Remembrance” will be free of incest, which starts development in 2023.