The Pretender (Animorphs #23)

Someone's looking for Tobias. Someone who says she's his long-lost cousin. Tobias isn't sure the person is telling the truth, but she's really nice, and knows a lot about him. And what she tells Tobias definitely gets his attention.

It seems a lawyer has discovered Tobias's father's last will and testament. So, Tobias needs to attend the reading. His cousin even offers to go along. But something just doesn't feel quite right. That's when Tobias and the other Animorphs decide to do a little checking on this "cousin." And what they discover will change the rest of Tobias's life...