You are both right in your own way. Personally, I believe that in this case, biology "goes to hell" and life itself takes over, and it tells us that these two have considered each other brother and sister since early childhood. And this relationship, this strong emotional bond, their feeling of being one family, simply cannot be erased by any revelation that they are not related to each other.
On the contrary, this new knowledge should empower them and give them far more strategic advantages based on mutual trust and knowledge of each other than any of Kane's affairs on the side. At least, that should work for our Dom side. And if you multiply all this by their romantic feelings, which are beginning to blossom between them, then it should be a killer combo for anyone who stands in their way.