Thank you for responding. To help both girls deal with the stress of their absentee mother, please assure them that their mom loves them and always will. But sometimes people do not make the right choices in life and it hurts others. Have the girls make an I Love You card for the mom with lots of pretty decorations. Explain that the mom will receive the card but may not respond. Also, the girls can keep a journal and write down all of the things they would like to tell their mom so when they do see her, they can reflect back in their journals. Writing thoughts down on paper always seems to help. They can draw pictures of things that they are doing and would one day like to do with their mom. The journal should be somewhat fancy so that the girls realize that it is special.
Also, you may want to mention to the dad that the girls get some type of counseling so that they can learn intervention and coping strategies in dealing with the absentee mom.
Hope this helps.