Abomination:Girl caught elder sister sleeping with their father

Things like this have been happening for long, but with the social media it,s not easy to hide anymore. A lady was caught her elder sister and her dad in the act is considering spilling the beans.
     There. Is serious fire burning in my family. At the moment. My mum had a child who is 5 years older than I am, before. She met and married my dad.
    Many don't even know that my dad is not the father of my step sister, about two months ago, I caught my dad and my step sister having sex in my parents bedroom.
    I have warned them but it is like. They are now going to hotels to continue their incest. I am really confused and angry, I don't know if I should tell my mum.
    I can't sleep with this burden on my heart but then, I don't want to destroy my family. Please what should I do, I need your advice