Need some feedback on a question regarding my fiance's children?

I am 49 and have enough wisdom and experience to usually resolve my own issues but this one is a bit perplexing for me. I have been divorced since I was 29 my daughter is 21 and in college. I met my fiance a year and ago and our wedding is this year. He has two children 17 and 18. His first marriage was bad, his wife was very hostile and abusive towards him. His children know of me, and he's told them (I overheard the phone conversation) he wants them to meet me and it's like they have no interest at all. I have tried to include them in Thanksgiving/Christmas or whenever they want to come by still nothing. My fiance told me that his ex wife told him "I don't want that woman around my kids" which I thought was terribly ugly and immature, but I suspect that may be why I haven't met them. I've been trying to tell my fiance that I don't feel that my first meeting with them should be on my wedding day, who wants the extra pressure of how they're going to receive me on my wedding day? I also feel bad that the ex wife is being so petty about me being around them. I want them to feel comfortable with me, I'm a great, funny, down to earth person so I thought if they meet me just once they'll see for themselves, but every time my fiance tells them to come by they're unavailable. They barely call him so maybe I shouldn't take it personal, but I wanted everyone to feel at ease.