Sindy Dietz may be Vera's mother, but she doesn't have much of a presence in the book. Since she left when Vera was twelve years old, Ken and Vera have pretty much taken care of themselves without any input from Sindy. In fact, she just sends Vera fifty bucks on her birthday each year and that's it for their relationship.
But what's left in Sindy's absence is the ghost of a mother and wife. Vera and her dad are haunted by Sindy's decision to bounce and a key step in their coming together and starting to really live is dealing with their feelings about Sindy abandoning them and really confronting their loss. When they go to therapy, they come to peace about Sindy's absence in their lives, and as this happens, there's no longer a Sindy-sized hole in their family.