You mean sisters? Or you were just trolling?
I meant brothers, not sisters. Obviously, two women can't be brothers, but my point was that their "actual" relationship is whatever you set it to be. You already know this, because you said it in the post which I quoted.
Yes, there are some places in the game where the dialogue states a specific relationship, no matter what you've entered, but those are errors in the programming, and all of those mistakes are supposed to be corrected. Going forward, the game text should always use the relationship terms which you input when prompted. And if there are any future mistakes, they'll also be corrected.
So, again, the "actual" relationship is what you say it is. If you want to make them brothers, despite this being biologically incorrect, you can do so.