Harvey's Hideout by Russell Hoban

In Harvey's Hideout, written by Russell Hoban with pictures by Lillian Hoban, Harvey and Mildred muskrat have trouble getting along, as brothers and sisters often do. They each complain about the other and pretend they don't need each other, but they end up discovering that sometimes it's nice to have a brother or sister to spend some time with.

Anna thought the story was very cute.  She was upset at times with how they treated each other but was please in the end