AMIBU�to want to know more about the back-story of my STB Step-daughter?

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Disclaimer.
Yes, I m a bloke, a decent one I hope.
I try. Just want some advice.
Getting absolutely nowhere with male friends on this. Useless gits!
A good long-term female married buddy suggested I try here, as she is stumped.
I have had a look around on various threads and tried to learn the lingo, and I think this is the best place to post. Flame away if you feel like it! But I would like some advice if poss.
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Next year DP and I will be getting married. But one subject has always been off limits. Always! For years It is starting to drive me crazy. AIBU???
Just so you know, I love my DP totally, and trust her with everything.
Her DD is nearly 17 and quite the modern young lady and we are good friends, but I cant possibly raise this issue with her!
I adore my DP and we have always been very open and honest about everything, including talking about ex-Ps, open about money etc. parental difficulties, getting STI tests before embarking on serious relationship etc. I feel completely comfortable talking with her about everything but

The subject of the parentage of her daughter is never to be spoken of.

We are getting married next year and I feel I should know. AIBU?
I have, for several years, happpily fended off questions from friends and family about this, but with the marriage impending, it is becoming increasingly difficult. I dont really care what other people think, I love DP, and am committed to her.
I realise it maybe something dodgy, like rape or ??, but I will love her anyway.
I have not pressurised her on this, but feel it is something I need to know about.
Who is the Dad? Alive or dead? Is he going to show up some time? (I can deal with that, but I dont even know if he exists!?!?!!)
It is starting to eat into me, and I do not want it to become a worm in my brain and bug me forever.
I just feel, if we are to be married, shouldnt we know everything about each other?
Or am I being an insecure idiot about this?
Thanks,

Friend of DH family is like this surrounding her DC father.

IL had pleasure of being round her when she was really drunk and emotional looking at old photos - very out of chacter at least since she became a mother. It's obvious who the DC father is.

She went on and on about how selfless she'd been - apparently they were dating pretty serioulsy but she'd never told him she was pg walked away from the friendship group and entire industry because he was so talented and she didn't want to impact on his future career.

She told her parents she had no idea who the father giving impresion she was sleeping around a lot at the time – they were at time hugely concerned and they have given her a huge amount of support over the years at times at expense of their own health.

The guy has apparently gone on to be very successful is married with DC. She believes he has no idea still and she doesn’t remember telling my IL.

I'm a bit about the desion even though I don't know the people involved very well but the DC seems unconcerend at present at mid teens age. I suspect Mother's family would be very upset by the lies and who know the impact on the father.

My IL really wish they didn't know anything - MIL especially is worried she'll let something slip and they really don't want to be involved.