I read this book sometime between 1977 and 1979. At the time, it would have been classified as a "thriller" type of book, along the lines of a Stephen King type of story. I *think* the author was a woman. The jist of the story was about a family, Mom, Dad and two or three kids. They adopted or somehow took in a very young child, a girl. Near the end of the book, an analogy was made comparing the little girl they took in to a cuckoo bird because one by one, the family members started "accidentally" dying, it the comparison was made because with cuckoo birds, the mother bird puts/lays her egg in a nest of a different kind of bird. When the baby cuckoo hatches, it kills/pushes the other babies or eggs out of the nest so it is the only one left and therefore doesn't have to compete with the others for food and has all the momma bird's attention.
One other thing I remember regarding the subject of this book is that, near the end, when the Mom and Dad were discussing the possibility of having another child, the little girl had mumps and "purposely" infected the dad, and apparently he became sterile from that so they couldn't have anymore children. I don't remember how the book ended - I tend to think the little girl killed the dad, too, eventually, and just she and the Mom were left and that was how she wanted it.
I know this is a little obscure but I thought if I can find out anywhere, it would be here! :-)