SOLVED. Y/A Fantasy trilogy. Cousins born on the same day at exact same time, look for their parents in fantasy worlds. [s]

[River]

I read the first two books of this trilogy when I was in middle school, so about 2010-2013. They’re Y/A or children’s books about these two cousins, one girl and one boy I think, who happened to be born at the exact same time. They live with one of the kids dads. The cousins parents were also identical twins, they’re dads being brothers and their moms being sisters.

SPOILERS

When the dad goes missing, the two kids have to find him. I can’t really remember how they get there, but they end up in a fantasy world. They end up in one place where it’s a giant underwater city, including a school/college of some sort, where they breathe under water and are there for a little while, and in another book I think they’re in a place where everything is in the sky (possibly floating islands and things like that). I also remember that one of the cousins parents died in a car accident before all this started and the other cousins mom either disappeared or died as well, leaving them with just one parent left. In the book they start to realize that maybe their mothers were from these fantasy worlds?

I’m sorry I can’t remember much about the books, as I never got to finish them.

[Marisa]

Based on my Google searches, I think this might be the Crystal Doors trilogy by Rebecca Moesta & Kevin Anderson. The first book is Crystal Doors.

One of the reviews on this book says: "Gwen and Vic are our two protagonists, and they are also "twin" cousins. This means that their dads are brothers, and their moms are sisters."

[River]

Yes that’s it! Thank you so much!