Mortal Sins is the fifth novel in the world of the lupi series. Note there are several short stories in this series that appear in various anthologies.
In this book Lily and Rule are in North Carolina, near the Lediolf clan's home, to pickup Rule's son Toby. Rule discovers some dead bodies and their family visit turns into a death magic investigation that endangers Lily, Rule, and Toby. Add in complications with the Lediolf clan and what was a settled custody battle over Toby and the story gains depth and momentum. It's another one that's hard to put down. Highly recommended.