I think the author knows that Elf is the best character by how many wife points she's racking up, while Sagiri's relevance to the story is her paper-thin character and titular name.
You can't marry your sister anyway so he should just marry Elf so the three of them can live together. Though I'm not sure of what the law says about stepsisters.
@Ovdose No law to prevent step siblings marriage. @flxsemi No matter how much waifu points Elf raking up, based on author track record you know author will make Sagiri end girl.
He/She even prepared the step sibling setup right in the beginning of the story compared to OreImo.
I dont know how to reply to posts but to @Bakaman there's an Elf spinoff series written by the same author (and is a better read imo), which scares me because it lessens the probability of Elf winning in the canon story.