AITA for dating my best friend's dad

This is my little sister's account (she's 18 btw), since Mia or anyone she knows doesn't know this account.

I've known 'Mia', my best friend, for about 6 years. We met in college, when I was 17 and she was around 19. Now I'm 23 and she's 25. Mia and I have always gotten along very well, except for a few petty arguments here and there, but nothing big enough to break a friendship.

Mia's dad (now 53) and mom (also 53) had gotten divorced when she was 15. So, long before I came on the scene.

When I met Mia's dad for the first time, it was Christmas time. She'd invited me over to her house for christmas since I had no family close by. (I was studying abroad and my college fees were high, so travelling back home was a biiiiiig no.)

Mia's dad and I DID NOT like each other romantically AT ALL at that time. I need to make this very clear, for a reason that you will see as the story progresses. In no way, shape, or form, did he or I ever flirt or look at each other in any sexual/romantic way.

Around fall two years ago, I started getting a small crush on her dad, but I didn't act on it at all. I told Mia, because at that point we had known each other for so long, that she could pick up on when I was hiding something very easily.

She found it incredibly funny and we laughed about it.

Then, during summer break last year, her dad asked me out. He was very respectful about it, and I said yes.

I asked Mia if she was okay with it, and if she wasn't, I would say cancel the date.

She said she was fine, and she even helped me pick out what to wear and everything.

Everything was smooth sailing, and I was happy.

Her dad is a perfect gentlemen. He gets me flowers and chocolates and listens to my long rants about books and other such things, the whole shebang. My family is perfectly fine with this, after a little persuasion.

And then he proposed, a month ago.

Of course, I accepted.

That's when things started going downhill.

Mia's mom found out, through a mutual friend.

Mia's always been a mama's girl, so when her mother was unhappy, she was unhappy. Her mother said all sorts of vile things about me, saying I was a homewrecker (which makes no sense since they got divorce before I was even in the same continent) and I was dating him 'just for money and a green card', which stung.

Mia's dad, now my fiancé, knows that this isn't true, but Mia got riled by it.

She's become cold, and she told me that she knows I only became her friend to get close to her dad. She said I broke girl code, and she's managed to convince most of our friends that I'm a horrible person.

So what should I do? And AITA?

Edit: For the people saying the marriage was rushed, maybe it was, according to you. But in my family, its completely normal, so I didn't think much of it.

And for those who are saying I need therapy, yes, I have, in fact, gone to therapy, and am still going to therapy. And yes, my therapist was slightly alarmed when I told her, but you can be rest assured that this is not due to daddy issues or any sort of mental problem. I just like older men, and I'm old enough to know who's good and who's a filthy pervert.

Mia's dad was and still is, extremely respectful, so please please don't make him out to be a bad person. I understand that its not a conventional age gap.

In adittion, I feel the need to reiterate that I would have ended the relationship before it even started if Mia had said she didn't like it. Even if we were dating, I would've called it off. But this happened after the engagement, and she had no problem even then, until her mother did.