Is there anything that I can do to help my ex's family accept my daughter?

[Anomyous]

My oldest daughter (from a previous marriage) grew up knowing my ex as her dad because her dad was not in the picture. She met him when she was 3 and he took care of her as if she was his own His mother was very good to her and treated her like a grand daughter. It was not until he returned from serving over seas that we had a daughter of our own. Things did not work out between the two of us and we went through a bitter divorce. Even though our feelings for each other changed I made it very clear that I would like for my ex and his mother to be involved with each of the girls. They have visitation on the weekends. His mother comes to pick up his biological daughter but totally ignores my daughter. When his mother comes to the house she always talks to her biological granddaughter while totally ignoring my daughter. When it is possible I try to have her with family or friends to avoid being totally ignored and hurt. However, that is not always possible. Is there anything that I can do to make this situation better? How can I help my daughter to understand that its not her fault and that she is just as special as her sister? Any advice would be appreciated.