i think my mom has a secret daughter?

So I am a 19 yr old girl.Since I can recall my sister used to always play around that my aunt was my sister,but I never thought anything about.My aunt would even tell people that we were sisters(think I got used to all of)Ibegan to wonder why they talked like this. When I was 14 I asked my but we ended up fighting and my dad got involve.They both rested and said my sister was crazy.I wasn't starting to leave it like this so I asked my mom to tell me the truth.She said when she was 15 she had my aunt/sister,but she gave her to my grandmother.Till this day I still feel real hurt.

And then last week I used my dads phone to call someone because my phone was charging.My parents share the same FB and they would care if I checked their FB. I realized that woman was calling their FB but I didn't answer bcuz I didn't know who it was. I imagined it would be an aunt from Mexico that I've never met or something. it said "hi daddy, I really miss how are you". I was so shocked I felt numb. I didn't say anything to either of them. so yesterday my dads phone was in the kitchen table and my mom was using hers. I caught that same girl pop up.. I looked into it and my mom saw me so she ran and took the phone out to me. She told my daddy that I grabbed the phone out of her hands so I got in trouble, so I got so mad at both at them because she was lying.I can't my can be such a person.I am so hurt Bcuz I've never met my family from Mexic.Sometimes I feel like my life has been a lied to. Thanks!