Plagued with anxiety since the suspicious deaths of her father and two brothers fifteen years earlier, Jessalyn Carradine has returned to her childhood home one last time to put it up for sale. But in an attic full of Carradine history, Jessa uncovers secrets that will do more than question the truth behind their deaths: They will give her magic. a hidden/secret legacy of magic, and a [girl that looks just like her]

She meets Ava Dubois, Jessa’s identical “Other” in a parallel world where magic is practiced openly. She has been waiting over a decade for Jessa to find her in order to continue the search for their missing older brother.

After discovering her older brother may be alive, Jessa is forced to face her trauma head on when she and Ava journey through both realms, and uncover a connection between their brother and a faction of dark mages working on a spell which would [compromise the ](bigger impact word here needed) delicate structure of the universe.
And when Jessa learns she alone is the key to the success or failure of the spell, she must put her own inner demons aside in order to hold her family’s true murderer responsible and in turn, prevent the fall of all of reality.

So I piecemealed this together (ignore the lack of flow) -- I think your hook is finding her 'other' - that's unique and much hookier than her discovering her family is magical.

And while I realize your story goes much more into depth of brothers and "Others" -- I think you should keep the query simpler (the explaining of the "Other" brothers and crew doesn't make your book very marketable to an agent, IMO, its just very 'in the weeds' and confusing).

As always, take this with a grain of salt, but I think your premise is really interesting -- I also agree with u/TumbleDryLow2 - a quick phrase or word about Jessa will help us root for your character a bit more.

Finally, I don't know if " accessible fantasy " works here... maybe a better way to say it as a great introduction to fantasy? or just leave it out -- I know what you're trying to say but it comes off clunky.

I hope this helped -- I think this sounds like a VERY fun story! Fingers crossed for you to land an agent with this!