It certainly looked like that, and it feels similar to a manga I read where sibling like the same guy, the girl marries the boy but has died and so her brother ended up with him instead.
But in my opinion, I think that would only be good (it being yaoi) if everything was shown from the boy's perspective. Otherwise, it'll just feel like "why bother making the girl the main character if it's him who is the actual main character? Those sort of formats must be used as omake or epilogue when the main character has done his part!" Kinda thing. Right? Main story, boy perspective, yaoi or shounen ai genre, then his sister's perspective as an omake! Otherwise... Well, it's like making a main character out of a side character that it feels iffy esp. Since it's a one shot.
The last chapter should have been yaoi