Why would Liesel be so interestedin contacting her mother? why would the Hubermann's avoid conversation about Liesel's mother ?

Liesel no longer lives with her mother. Her father was arrested and things didn't look good for mother. Her mother sent her to the Hubermans because she could not ensure Liesel's safety. I think the Hubermans knew there was a good chance her mother was killed or taken by the Nazis.