So many miserable, and quite nasty buggers on this thread @Candlemascandy, I'm sorry you're having to deal with this.
I don't think anyone who has a "normal" relationship with their parent wouldn't be ever so slightly put out by this situation - I know I would be, and I'm generally the type that doesn't enjoy weddings much at all.
She's your mum. And you're being excluded, they say for financial reasons but are then including her partner's child. It might also be just to tie up those loose ends before the inevitable happens - but I'm sure it's sowing those seeds of "are we even going to be remembered in mum's will?".
I think the best you can do is tell her that you're hurt you're not invited, that you want to come and will cover your own costs as it's important to you that you're there.
Unlike the miseries I don't think it's wrong to be honest and open with your own mum that it is important to you.
@TopshopCropTop Because that’s not the message. The message is “our marriage is not about you please respect our wishes”
This isn't the marriage. This is a wedding a singular, one time event. This has the potential to really impact the rest of OP's mum's life, when the hurt this will cause comes back to bite her.
Either it's completely unimportant and any Joe Shmoe can be used as a witness and no celebrations afterward, or there is an element of important hence why one child of three has been included and a nice meal will happen afterwards. Why even tell anyone if it's not important?!
Seriously some of you. It's not out of order to be upset when you're excluded from a life event of your parent. No wonder so many people are feeling so isolated and lonely these days when the advice they're getting is that nothing is their business and that they don't have the right to have feelings about their own family.