I can't remember book series- horses?

Okay so when I was in elementary, 20 now, I started reading this series about twins (girls? maybe one girl one boy?) that showed horses and solved mysteries.

I loved them and for the life of me can't remember the titles.

I've googled and googled and still can't find them.

It was a whole series about twins that showed horses and solved stuff in the barn along the way.

It wasn't little kid crimes like loosing a pen, but I think there was a fire they got caught in that was intentionally set.

I've already gone back and asked, don't have them anymore and different librarian.

Not Mary Kate and Ashley.

Series was modern enough at the time.

They were probably written between 1985-2000