Theory: Maru’s room used to be Sebastian’s room

There’s only two bedrooms in Robin’s house. Robin specifically mentions that she built her family home herself. When she met and married Demetrius, she already had Sebastian from a previous relationship. She built one bedroom for her and Demetrius, and one for the young Sebastian. Some years later, Maru came along, and for whatever reason, rather than building an addition to the home, Sebastian was moved downstairs to the basement. It’s unclear if he and Maru shared the room at any point.

This also implies that Robin and Demetrius weren’t anticipating any more children, and that Maru was a (happy) accident. If the couple had been experiencing fertility issues but Demetrius had always wanted a child of his own, this would explain why Demetrius shows so much favouritism to his blood-related child despite raising Sebastian from toddler-age. Apparently this favouritism would extend to giving his daughter the bedroom and moving his stepson, who previously owned the bedroom, into the basement.