AM I being a BIT@&?

Here's the situation. I've been intimately involved with a man who has 3 older children for nearly 2 years. At Christmas he said it was important to me and himself to go to my mother's house Christmas eve, which he did and enjoyed himself. He tells me daily he loves me, and hopes to marry me. Here's where it gets sticky. On Christmas he went to his sons' house with his 2 older daughters and the ex-wife was also there. He's told me that it's a tradition his children expect at the holidays, and he has no feelings for their mother anymore. The son asked him if I would be accompanying him. He never asked me to go with him, why? In the past year, they've celebrated a daughters wedding, a graduation, granddaughters first birthday party, etc...none of which he's asked me to attend? What do you think is the problem?