Not a big fan of that backstory myself, mostly because of the comparison you already mentioned, but I think it was used more as a mean to add a (half) sister in the menu.Agree, it doesn't matter anymore. It is important that she sincerely loves us. Although at first she fell in love with us because of the MC eyes (MC has eyes similar to his father)
I must admit that I was shocked by this news when I heard it for the first time. And it pushed me a bit away from her.
But there is no need to worry about what was in the past now. Past is past.