I Caught My Elder Sister Sleeping With Our Father

Things like this have been happening for long, but with the social media it's not easy to hide anymore. A lady who 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 five years older than I am, before she met and married my dad. She has 3 of us for my dad and we have been living peacefully together for about 19 years now.

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 s*x 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 well with this burden on my heart but then, I don't want to destroy my family.