Op, I really sympathise with you.
While pp are right, I do feel you could do with a little more understanding.
The problem with immediate family relationships is that they are 'shared' in so many ways and so it does affect us emotionally when we realise that someone else - especially a sibling- doesn't see them in the same way. It can shake our sense of security within the family and leave us questioning whether the love we feel for people is reciprocated.
So, while I agree with pp, I think you're fully entitled to feel the way you do and the reaction both you and your mum have is completely valid - because in a family, we're all inter-affected by each other.
That said, of course the difficulty is in how you process the emotional shock and resolve your feelings.
It might well be that your sister is conscious her choices will have caused waves but not knowing how to deal with them, or being too scared to confront them, she may just be ignoring it. Does that sound like something she might do?
I can easily imagine her posting on MN worried out how her choices might affect her family. Of course, she'd be told to do what feels best for her and that's correct.
Its difficult to know why she's made her choice, if not impossible, based on your posts but perhaps she feels a sense of obligation to that side of her family. Perhaps also, she is trying to build and strengthen relationships with them. Maybe she even believes those relationships to be something they're not...Who knows, certainly not us or even you.
There's an old saying that a pair of siblings raised the same way will have had completely different experiences of their childhood and you don't have to look for on this site to see this borne out!
Your sister may well have been harbouring feelings for years that she has not felt able to share.
But, when it comes down to it, your mum, dad and you need to make peace with the rejection you feel.
I think you've done the right thing in choosing a safe, neutral place to say how you feel and get your hurt off your chest.
Try not to question things too much. I'm sure your sister loves you all - and undoubtedly knowing how much you are all devoted to her will be important to her. But she's clearly trying to show the family that she hasn't lived with how important they are to her too.
In getting married, she's also creating her family for the next phase of her life so I'd see her wedding as the way in which she's bringing other people in.
Love isn't restrictive, there's room for everyone. Try not to see this as a rejection of you but her way of embracing others and her way of honouring her dad and ensuring that he's with her on the day.