Firstly congratulations! And welcome to the board, and I hope that you're having heaps of fun planning what's supposed to be a fun day for you. I recommend you think of it this way; the only thing that can really go wrong is that you don't end up getting married. Everything besides that is just frosting, so just let it go and enjoy yourself.
Re siblings: my four siblings didn't come to my wedding, and that did something to me. I knew they wouldn't be there but I was actually expecting to get say a mobile phone text message from them, just quietly wishing me a good day. Nothing. It was such a special day for me and they weren't there even in thought, even as a sibling privately hoping the best for me. That switched something in me; I stopped thinking of them as people who care about me and realised that they're capable of manipulating my attachment to them to their advantage. They will use this emotional blackmail to draw me back, and don't see anything wrong with that. Since that day I have rejected that kind of 'love', and I don't want people who would treat us that way in our lives. I don't think they'll ever understand that what they're doing is achieving the exact opposite of what they were going for.
Re handling your mum: give her something to do with the wedding. She wants to feel involved, and as if her opinion is important. Ask her opinion on things - you don't have to go with her suggestions but it will make her feel valued; just on the details that mums seem to get hung up on; cake, dress, hair, makeup, music, menu, that kind of thing.
My mum has done the flowers for every JW wedding in her circuit for twenty years so it was an easy way to get her into it, although I found that she was far less involved than I thought she would be, and didn't really give a stuff. It turned out a couple of years later that they were upset that my getting married was 'presented to them as a fait accomplis'; I hadn't asked them if I could get married, I just told them I was. I was in my thirties and hadn't lived with them for ten years, but they still wanted a say in my life. It was such a warped situation and the disfellowshipping possibility came up within seconds of my telling them that I was engaged, so there was never an appropriate time for Mr Frass to 'ask for their permission' as people like my parents still consider it necessary. Apparently they didn't really get over that.