I didn't know BGS had the same author as sakurasou, and I've heard that the latter half of the sakurasou LNs are horseshit, so I really worry about the unadapted novels of BGS
New theory, dude was hit on the head twice as a child, once after becoming erika's "brother", once after becoming friends with study girl (nvr remember her name) and promising to marry her, and here we are. he's the brother of his fiancee and also the fiancee of study girl. or multiple...
what are the chances that scholarly love interest girl (don't remember her name) turns out to be blood related, so she's out of the question? or she's also a "sister" and she's engaged to him already without either knowing it?