For the older girl:
The Winner’s Trilogy, by Marie Rutkoski, starting with The Winner’s Curse. All three books are quite good, with a main character who is actually very smart, if flawed.
The first in the Great Library series by Rachel Caine, Ink and Bone. I haven’t read further in the series yet, so I can’t say if it will hold up, but I think this has a lot to offer a Harry Potter fan, though it is for an older reader.
The Denton Little duology, by Lance Rubin, starting with Denton Little’s Deathdate. Very very funny fantasy/sf that is perfect for older teens.
And for more traditional fantasy, The Cursed Kingdoms trilogy by Emily Gee, starting with The Sentinel Mage.
For the younger girl:
The Tillerman Cycle, by Cynthia Voigt, starting with Homecoming.
I Capture the Castle, by Dodie Smith. A great classic epistolary novel.
And the related The Montmaray Journals, by Michelle Cooper, starting with A Brief History of Montmaray. It’s about a fictional kingdom during WWII. Really good, I think, for people who are interested in history.
The Ruby Oliver books by E. Lockhart, starting with The Boyfriend List: 15 Guys, 11 Shrink Appointments, 4 Ceramic Frogs and Me, Ruby Oliver. Just a spectacular series for younger teens that explores dating, friendship, families, and school.
Seconding Uprooted and the Lockwood & Company novels, too.