My husbands ex is destroying our relationship with his son

I Come from a family whose prents are still happliy married so I have no experience and absolutely no idea how to deal with my husbands ex and how she uses my step son as a pawn to get back at his father, my husband. HE lives with her most of the time as we live 4 hours away. She has now refused to bring him half way to meet us saying that if we want to see him we need to cone get him and then take him back. SHe tells him lies about us and its breaking my heart, i dont know how someone can put their own feelings before their childs. He has a little brother and sister gere who adore him adn we have always done whatever it takes to havr him here but its getting to the point where we cant meet her demands. I dont know how to explain this to my kids anymore, I dont know how I can keep being the bigger person when she just stoops lower and lower. MY step son is 13 and i feel like he is starting to be heavily influenced by his mum more so than before.. i dont know what to do to try and fix this anymore! Please help!