Conspiracy, The (Animorphs #31) by K.A. Applegate - Vintage Paperback

Jake's great-grandfather has died. It blows the family away—especially Jake's mom. So Jake's parents decide they should all travel to the great-grandfather's home to help with any arrangements. In another state. For three or four days.

This is a very big problem for Jake's brother, Tom. He can't be away from the Yeerk pool for that long. So Tom refuses to go—and his father insists. But Tom's Yeerk will do anything to survive. Including making his father less of a problem.

Now Jake, the other Animorphs, and Ax have to find a way to protect his father without Tom discovering their secret. And without having to choose between his father's life. . . and Tom's.