Weird situation

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Okay, so I watched this episode on House once about a couple that through a series of events found out that they were actually half siblings. Here's my question. If you are in a serious relationship with someone and you're totally in love with them and you know that there won't ever be anyone else for you, nor do you want anyone else, and you find out that you and your significant other are half siblings, what do you do? Do you leave the person because it might be weird between you to? Do you just become friends? Or do you just figure that it didn't make a difference when you were together and didn't know so why should it now? Does knowing really make a difference?