@BettyBurntBuns It's clear that you haven't been invited. It's likely your mum doesn't want to tell you that herself, because it's between you and your sister, not between you and her. I don't think there's an instance in the history of the world of someone blocking and ignoring someone else, and yet wanting that person at their wedding.
On some level, you must know that you haven't been invited. Why do you need one of your relatives to spell that out for you? I half-wonder if it's because you already feel rejected, and you want to make yourself feel even worse by hearing it again. People are often their own worst enemy, and I wonder if that's true for you.
Self-worth starts with how you feel about yourself. If other people like you, that's lovely, but the most important thing in the world is to like yourself, and it really doesn't sound as if you're in a place where you do right now.
Forget about the wedding. Forget about the number of friends you have. What do you like about you? There must be something you like about yourself, even if it's a small thing?
If there are things that you don't like about yourself, can you change those?
For example, I decided I didn't like how fat I was. So I lost weight. I thought I wasn't very interesting. So I spent some time looking into potential hobbies, found one that seemed like it would be a good match, and I got into it.
No one gets to judge me apart from myself, and if I'm my worst critic, I need to balance that out by being my best advocate too, and telling myself to do things to make me happier and healthier. When you care about yourself, it becomes easier to let other people, and to build connections with them.
You can be your own best advocate too. And that's where the self-worth starts. With you, and your own feelings.
You are worth so much more than you think.