Appropriate to ask fiance's kids to be in our wedding?

He has 2 girls (only 5 and 10 years younger than I am) and 2 boys (same ages roughly) from 2 previous marriages. I have never been married. I only have 1 or 2 friends I would want in the wedding party. I don't think my fiance would have more than that either. How would I make them feel special and involved? Another question, is it informal to make this wedding a big deal since it's my first, but his third? I would like to have a nice typical wedding but like I've said, he's done it before but almost twenty years ago (his first wedding at 18 and second at 23). I wonder what people would think/say. His family can be a little snooty. I realize I shouldn't care, but I do. Any advice?? Thanks!