My mother gave the kids of this long lost relatives about $200 in red packet money but they never invited her to the wedding banquet. The parents called her back to thank her profusely and I think they were very embarassed that they didn't invite my parents. When I heard that my mother gave them that much, I was mad as hell. I mean, c'mon!! Who gives $200 to distant relatives. Man....
I always give out a token gift to the couple even if I decline going to their wedding or whatever. Like PC and JJ say, it's only good manners to do so and to wish the couple happy success in their new lives. Doesn't have to be much, it's the thought that counts.