My mom (F/58) has eloped unexpectedly.

I'm 33/F, and I'm the oldest of 6 adult children. Our parents have been divorced since 2012. My dad (63) isn't interested in remarrying, and for a long time, I didn't think my mom was either. She's had a few short-lived relationships here and there, but nothing really serious or lasting.

Then, yesterday, she tells me that she's married a man who she's been dating for around 3 or 4 months. Apparently, she had met him before in the past, through a mutual friend or a friend of a friend or something. So he was somehow peripherally in the edge of her social circle, but not like a good friend or someone she really knew. They started seeing each other in June, and sort of fell for each other really fast. So earlier this month, they went to the courthouse and made it official, in secret.

Even though I love my mom (of course), and I want her to be happy, my siblings and I are befuddled by the whole thing. We didn't know she was seeing someone. He's never met us. There was no wedding. And yet, now he's our new "stepdad" and our mom's husband.

I've looked him up on socials to try to learn about him, and I didn't see any obvious red flags. He's gainfully employed, went to college, and seems nice enough.

Still, it all seems a bit off to me. I don't really understand why they rushed into something as serious and legally binding as marriage, after just a few months. I'm tentatively accepting, but also a little worried for my mom. What would you do here?

tl;dr: my mother has married a man after only 3 months and our whole family is bewildered.