There’s a slight chance my girlfriend is my half sister!

I was out with my dad and a friend of mine when my friend let slip out the fake name I go by when I don’t want girls to know my real name. My dad laughed and said he use to do the same thing and then told me his old fake name. The fucked up part is its the same name of my girlfriend’s supposed dad, which her mom had a fling with while on vacation, but was never able to contact afterward.  There’s a slight chance my girlfriend is my half sister!