The Grammarians by Cathleen Schine

This book is on the New York Times Notable Books list this year and it is a really fun read.
Identical twins Laurel and Daphne Wolfe, both named for the same minor Greek goddess, shared everything: a womb, a language known only to themselves, the red hair, and the confidence that comes with knowing that you will never eat lunch alone at school. They also share a love of English that sets them apart from much of the world but binds them further together.
They are head-over-heels in love with words.  They love them and collect them as other kids would amass sea glass or baseball cards.
This is also about how shared DNA can foster a ferocious internal rivalry, while it renders the pair nearly impervious to attack from the outside world. When the big rift does descend, it is at least initially devastating. They find separate but laudable outlets for their language obsessions, with Daphne writing a somewhat hectoring grammar column, and Laurel who writes poetry. Words are exchanged, but fewer and more rarely. As they say, the reason that our families can push our buttons is because they’re the ones who installed them.