I [20m] discovered a secret half-sister [15f] which proves my Dad lived a double life? My Mom doesn't know

Sorry if this is a bit incoherent my brain has been mush for the last week

So this one will need some explaining but the only other person who knows about this is my sister [19f] and obviously the "half-sister" [15f] in the title, and I'm the oldest person and could really do with some advice because obviously this is an absolutely whack situation and I've been freaking out for the past few days with the responsibility of it.

So my (real) sister and I are pretty close and go to the same college (I'm a year ahead though obviously). Last week my sister comes to me and shows me this message she got through on her facebook from some random kid girl saying she's her half sister, and then basically followed up with a load of details about our Dad to "prove" the she's his daughter too.

So for context my dad walked out on my mom in 2016, when me and my sister were still in high school, so we had to deal with the fallout from all that and tbh I don't think mom ever got over it. Dad did try to reach out to me and my sister separately a couple times but when he refused to give us an explanation we both vowed to never see him again.

But now ?? We both did some internet stalking of this "half-sister" who contacted my sister and she's from our state and knows lots of stuff about our dad. But she's FIFTEEN YEARS OLD, and says that her mom and OUR dad are happily married.

Is this possible?? Could dad have somehow been living a double life?? We've tried to scour our memories of our upbringing and like, he worked long hours and did lots of business trips but nowhere near like 50% of the time so I can't imagine how he'd keep some entire other family running. Is this just a sick prank?

Should we reply to this girl? My sister hasn't replied yet because she doesn't want to give her the attention/satisfaction but she did say in the messages she wants to meet us.

Am I going crazy? Could this be real? And the big question that my sister and I have been absolutely freaking out over the last few days -- do we tell mom?

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TL;DR: A "half-sister" contacted us out of the blue but dad only left mom 4 years ago and this girl is 15, meaning dad must have led some kind of double-life if she's telling the truth

No flair because honestly idfk what this would come under

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EDIT: One of the commenters brought up whether the OTHER woman i.e. this girl's mom knows, and I'm freaking out even worse now. What if she DOESN'T? My sister and I are the only adults in this entire situation, and our actions at this point could wreck an entire family??? Like I hate my dad if this is true but literally I don't know what to do now even worse fuck

Submitted January 30, 2020 at 01:22PM by secondsister20 https://ift.tt/2S4SW6f