Families Are Complicated

I'm married to a hot woman and should be satisfied. I was and should have ignored teasing and flirting of wife's younger "sister" My wife is 35 she has a sister who is just turned 20.

My wife's mum is nice enough but for whatever reason my wife and her are constantly at loggerheads probably Roman Catholic Churchy things.
The younger sister Mia is always at our house.
My wife can't have children which I can't say bothers me but we don't fuck that often either.
Mia her younger " sister" is shapely a tictok novice. Since lockdown Ive fucked her countless times.

Here's the Confession and total shock.

She's had an contraceptive implant and talking about something different, she mentioned that her "Mum" insisted ! I said that is good of Dee to support contraception being Catholic etc.
Mia looked at me with alarm.
You don't know do you, I thought all the family know, Ami isn't my sister you fool
She's my mum, she had me when she was 15. But Dee brought me up.
She grabbed my cock so really I'm fucking my Stepdad, mum should concentrate on you instead of work.

Why do I think this is going to turn out wrong?