Remember that possessive adjectives agree with the NOUN POSSESSED and NOT the possessor (person possessing). Above, you will notice that the selection of the word is based off the two girls, but the FORM of is based off the items the two girls own.
makes "nuestras" feminine & plural
feminine & plural "backpacks"
makes "nuestros" masculine & plural
nuestro
nuestro
masculine & plural "books"
El padre tiene dos hijos. Sus hijos son bebés.
Mi hermana y yo tenemos un perro. Nuestro perro se llama Boris.