The author does seem to enjoy messing with people with the Yuri-ness between the sisters and I have in past said I wouldnt be surprise (and personally be ok with it) if he went yuri at end. But now thinking about it I think I have a better idea of what will happen.... The manga here is tagged slice of life and doesn't even have a romantic tag giving some hint or hope that the story could go the direction of either sister being introduced to a male character to have a romantic end... So I see this lovely yuri-teasing love between in-laws to continue to the very sweet end of this story...
And after the story has come to it's contracted solution... the author will post on her own the extra last chapters that go full Yuri and they live happily ever after too. Then you'll get a few H chapters too.