Camelot trilogy about siblings

I read this trilogy sometime in the early to mid 2010s and don't remember much about it. Here's what I do remember if anyone knows this series I would love if I could know I've been searching for months.
About two siblings, possibly twins. One is a girl and the other is a boy.
Single father.
Merlin is a comic store owner. (Possibly some similar type of store)
The siblings travel to Camelot with help from Merlin.
They have adventures in Camelot.
I believe that they meet their mother sometime in Camelot and she is an evil enchantress.
The cover of the third book has the boy and a golden cat of some type on it I believe. This is just my fuzzy memory so possibly incorrect.
That's all I remember I hope someone knows it I can't find it anywhere.

This could be "The Land of Stories: The Wishing Spell" by Chris Colfer. The girl twin's name is Alex.

>2 blackbean06: It's not. Sorry.

It could be Magic Tree House: Merlin Missions by Mary Pope Osborne

>4 blackbean06: No. Sorry.

Maybe the Camelot Code by Mari Mancusi.
Some details match up, though the MC's aren't siblings.

It might be. I'll read it and find out.

I believe that it is the Camelot Code and I had just been discounting it because the cover image had changed. Thanks for your help!

The Camelot Code

Touchstone.

Nevermind It's not The Camelot Code. *sigh* there are only two books, and they both came out after when I read the mystery series.

Well the second one did at least. I read this series 2015-2018. Trilogy. Opens on the main character finding an easter egg in a video game. In a large cave? Finds this secret and it causes the whole huge journey. That's all I have to add to my description of it currently.

Maybe The Otherworld Chronicles by Nils Johnson-Shelton?

The Invisible Tower
The Seven Swords
The Dragon King

>1 IronKitty: What age group would you say your book was targeted at?