Meeting his kids

My SO and I have been together for about 8 months now and are expecting our first child together later this year and I still have not met his kids. He says the situation is delicate with their mom and she is bound to get in their heads and make them believe he's being a parent to my kids and not them and I understood waiting until after the holidays by now I feel like it's just getting ridiculous. I have told him this over and over but he always just avoids the conversation but saying he has to go back to sleep (he works nights) or he just says he doesn't want to talk about it at all. Idk what to do anymore. Any advice?