Lee's Girl- DSD's mom lived by the same games (there was always something more important than seeing her dad) at Christmas and other family holidays. She would agree to let him pick her up at some specific time, then if we were "lucky" we'd get a day or two's notice that there was something else for DSD to do. Other times we'd find out when DH was already on his way to pick up DSD. Ultimately the games led to the end of the informal agreement. He got a lawyer, then a court order giving him these holidays on an alternating schedule (among a lot of other things that made her pretty angry). He didn't get any Thanksgiving, Easter or Christmas with DSD until there was an order. At first, DSD's mom bucked the order. The first Christmas DSD was "too sick" to come, and when the story didn't convince DH not to exercise the holiday time, mom brought DSD to the emergency room and had a battery of tests run. Even the hospital doctors saying there was nothing wrong with DSD couldn't convince mom to let her leave. It was miserable and I felt so bad for DSD who kept asking us (once we finally got her a couple days later) to tell her mom that she wasn't sick and to stop taking her to the doctor when she wasn't sick.
That really stinks that she wouldn't let your DSS be a part of your wedding either. I'm curious what part of the parenting plan could enable her to prevent DSS from being part of your wedding (assuming you scheduled it during a weekend that was already "yours"). I was really afraid of DSD's mom preventing her from being at our wedding given her history--and she did put in a minor effort, telling DSD that she should call and ask to go home before the wedding--but it worked out OK in the end. DSD told us that's what she was supposed to do but that she didn't want to.
I would definitely make an attempt to pick up DSS when the order says for Christmas, and every other weekend for that matter, and document your attempt. Our routine is for me to take a picture of DH knocking on ex's door with a timestamp, and when she doesn't answer, he texts her saying he's there to pick up DSD and he knocked on her door and no one answered. She generally responds admitting she's not home and doesn't intend to hand over DSD, so we've never used the photos in court, the texts have been sufficient to document her behavior. I've heard some people suggest filing a police report (not calling the police to the house, just filing a report) to document the attempt, but I've never tried that because DH is vehemently against it, afraid it will get him in some kind of trouble. My DH was somewhat against trying to pick up DSD when his ex already stated she didn't intend to follow the ordered time because the ~$50 in gas expense when he knew he wouldn't come home with his daughter seemed like a "waste" to him. I finally convinced him that he would be spending the money if he got his daughter so put it in the budget and spend it regardless, even if it was his ex's objective just to waste his money and time. If ex convinces you to not try the pick-up, she can go back to the court later and say she didn't interfere at all, but that you didn't show up! So always try. The sooner you try, the sooner she'll have no choice but to let it happen and the less likely DSS will remember a time without dad being a part of Christmas (or his life in general, it sounds like). And if she's refusing weekends in general, especially if she refuses Christmas to boot, I'd file a complaint with the court. It won't get back the missed time, but hopefully will affect the future.