Maybe the issue is this: your sister getting married might make you question your own perception of “progress” in life. But career, settling down, etc., happen at different speeds. Viewing life solely as “marriage after this age” would mean missing the whole story. Focus on what you want, not just because of someone else’s pace.
I agree that it might make one question the perception of “progress” in their own life, but what is the foundation of that perception of progress? Because sometimes the issue isn’t just about “getting married” or “achieving something”; we are confronted with a strange standard in our perspective on life itself. Is it really necessary to have such a schedule? If someone were to ask, “Why aren’t you married yet?”, does your answer make you uncomfortable? Sometimes you need to think in reverse to find true feelings.
So, could your sister’s excitement during this wedding process be annoying or tiring you? You know how sometimes seeing someone overly happy can make your own struggles feel more pronounced? Is the real emotion not jealousy, but rather you’re keeping a bit of distance for that reason?
Maybe thinking about so many details is exhausting? I mean, do we really have to look for a meaning in this case? Perhaps it’s easier just to be happy for it and focus on our own path. After all, not everything is a message.
Don’t you ever think: If your sister has committed to this process and she knew about the chaos inside you, you might feel like you’re stealing from her happiness? Sometimes, the things we can’t resolve in our minds become an additional burden in someone else’s story. That’s why deciding what to do with these feelings is also a responsibility.
How do you view your sister’s decision and her relationship from the outside? I mean, when you set your own feelings aside, do you think this marriage is really the right choice? Perhaps part of the turmoil is being stuck between your doubts and her happiness. It might be hard to say if those doubts are valid, but remaining silent if you’re wrong could lead to regret. Which one weighs heavier?
Maybe the real issue isn’t his marriage, but it feels like the balance between you two is shifting? There might be a fear that closeness could diminish, and that you will have less space in his life. So, think about what you’re afraid of losing in the relationship with this marriage? 
Perhaps there’s something in all these questions: As your sister starts a new life, it’s normal for you to wonder, “What is my role now?” But who decides on that role? Is there really something changing, or are you writing this “balance shift” drama in your head?
Could doing so much analysis be confusing your mind even more? Is there something clear in your feelings? It might be a bit complicated on the surface, but at the core, there must be a simple emotion like happiness, sadness, or anger; go down to that. Everyone is getting married, getting divorced, the order is changing; sometimes you just have to let it flow.
Maybe you don’t compare yourself to his life plan, but does the pressure from your environment stress you out? People tend to jump into “So when are you?” mode during such special moments. Perhaps the discomfort is triggered from that side, not even from your sister.
Everyone has tried to figure out the meaning of the situation because your sister is getting married, but maybe you don’t feel anything at all? An empty feeling, neither joy nor sadness. It seems like no one has thought of the event in such a colorless way. That’s a feeling too, isn’t it?
Maybe the issue is this: Your sister is closing a chapter, and you don’t want to close yours. For her, getting married isn’t an end, but it might feel that way for you. Or it could be a complete fear: "How can she take such a big step while my life stays the same?
Have you ever thought, “What is my role in this process?” I mean, not just about your own feelings, but how you can be a supportive big brother for him. Perhaps the confusion stems from not being clear about your role in that situation.
Is it really worth overthinking this much? Maybe you’re blowing it out of proportion by thinking too much about it. People are moving on with their lives, and your place isn’t going to change 
But isn’t the real issue that it’s not about you? I mean, for example, how does he see you in this new life of his? Perhaps there’s a concern of “how are we going to manage this” as your relationship evolves into something different. It’s actually a question not about yourself, but about him 
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It seems there’s a presumption that your sister’s marriage will completely change the dynamic between you. But what will happen once the wedding is over? Aside from a physical distance, does your relationship really need a new identity? Maybe it continues with the same intimacy, flowing along without anything dramatically changing. Sometimes the big steps aren’t as big as you think.
But for example, isn’t the assumption that “nothing changes” strange? If there’s something that doesn’t change, it’s that everything changes. Maybe there’s a bond that transforms little by little, but you just haven’t noticed it yet. That’s why you need to look more carefully; will it really stay the same?
Perhaps the problem is not that things haven’t changed or won’t change, but rather the pace of change? I mean, before you’ve gotten used to something, it might feel like it’s taken a completely different step. It feels as if your time and its time don’t align. This can be confusing 
Maybe the issue is entirely about the expectations coming from outside? They put you in a box, thinking, “Your brother is getting married, he must feel this way for sure.” Could it be that the confusion actually comes from the meaning that those around you attach to this wedding, rather than your own feelings?
Maybe the issue is not your sister, but how you want to write your own story? Could her marriage be making the deficiencies in your life more apparent? The wedding is just an excuse; I think you are questioning your own path.