How do I ask my mom the reason for divorce?

Im 20, my parents got divorced when my mum was 1 month pregnant with me and didnt tell my dad. She then moved to Australia while my dad stayed in England.
The first time I met him was on a Skype call at 7 years, and he moved to Australia when I was 13, and didn’t call him dad until 16.

Now, I have a half brother (same dad, different mom) who is exactly 9 months younger than me. I know that my dad and stepmom got married before he was born. Which means there are 2 options. Either, in the 17 months since divorce and brother’s birth, my dad and stepmom met, fell in love, moved in, got married, and had a kid. Or… he cheated on my mom and that was why they divorced. I honestly cant think of any other reason.
I have tried to ask my mom the reason but all she says is “we just grew apart”. That is a valid reason, if my brother wasn’t so close in age. There is a possibility that she is trying to save me from thinking bad about my dad, but it’s honestly worse to wonder and come up with even worse scenarios.
How on earth do I ask the question “did dad cheat and thats why u got divorced or is he just super quick with love?”.

Ps: My dad and stepmom are still together and very much in love. Date nights and all. I also love her. She is so great, acts like she is my real mom not just a stepmom, and is someone I can always go to for everything. So I know that no matter the answer, I will still love them.