Too many times in the author's experience as an Adoption Social Worker, birth parents would seek to place a new child for adoption but had no resource to help them explain to the other children in the home why the new baby was not going to live with them. The purpose of "But, Mama! She's My Sister!" is to fill this resource gap. This book is still as relevant today as it was when written.
The answer to this question is an emphatic, YES! In fact, all members of the Adoption Triad (birth family, adoptive family, adoptee - It is also a good resource for foster parents).
Children relate better to animal stories. Besides, I love cats.
I have met many of my extended family. I stay in contact with my half-sister (oldest of siblings), but not with my half-brothers. I am also in contact with some of my extended birth family.
The absolute best way to tell a child he/she is adopted is to start telling them from DAY ONE! Even if the child is an infant, include "But, Mama! She's My Sister!" and other similar adoption books in your regular reading. Will an infant understand? No, not at first. But you will be comfortable with the subject when he/she starts to ask questions. Keep your answers age-appropriate. A 2 year-old doesn't need the same answer as a 4-year old, or 6-year old.... The "right time" to discuss adoption is the first moments the child becomes yours and any appropriate time thereafter.