The Arm of the Starfish by Madeleine L'Engle
Technically this isn't part of the A Wrinkle in Time series. But it has the O'Keefe-Murry family in it, and chronologically, this falls right after A Swiftly Tilting Planet (Time #4), and about two more O'Keefe Family books before An Acceptable Time (Time #5, but also O'Keefe Family #4).
Spoiler alert: Even though the whole point of the book is that a new character Adam Edington is going to work for Calvin O'Keefe, Adam doesn't make it to the lab until about 40% of the book. When he does, he still doesn't get to work, but goes swimming with dolphins. Then a little bit of the starfish excitment, then more blah blah blah.
Also, Calvin and Meg, who are about 37 years old, have had 7 children by now. Poly (12 years old), Charles (10), Sandy (8?), Dennis (6?), Peggy (4), Johnny (2), baby Rosie a.k.a. Mary. Holy cow.
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