I'm not going to tell you that you should go ahead and participate in your sister-in-law's wedding -- I'm not even going to begin to meddle in family dynamics!
But I will talk generally about the bigger issue -- you're going to encounter lots of people in your life -- family, friends, acquaintences, even people you thought were your best and closest friends -- who won't be able to see past the disease. At least your sister-in-law was honest -- she told you that she was uncomfortable around you, that she didn't know how to act or what to say. More often people just drift away, or slam the door in your face.
And you are right -- you don't have control over her actions, or behavior. Only your own. Have you done anything that might scare her off? I know that lots of people when newly diagnosed find their diagnosis the only thing they can talk about. It will consume them, and they'll talk specifically and sometimes graphically about what is going on with them, and talk about old symptoms, new symptoms, possible new symptoms, the "what if's?", the bladder issues...all that. When people ask, "Hi, how are you?" -- rather than taking that as a conversation starter, and answering, "Fine, thanks, and you?" they'll launch off into a litany of all the things wrong with them that day.
And frankly, nobody wants to hear that. At least not all the time. Even close and dear friends. What they want to hear is -- "I'm fine", most of the time, and only "Gee, I need some help" when you really need or want some help, or an ear.
And it's not always easy to discern the differences, there, when you're deep in the swamp and alligators (MS related stuff) is coming at you from all angles.
I don't know if you fell into that trap -- the "I'm awful and here's what's wrong!" trap, every time you talked with her. But if you did, that might be adding to her discomfort.
So -- maybe...and I guess I am stepping into family dynamics a bit, here...maybe now that it's been over a year since your diagnosis, you're more comfortable with it, and don't talk about it as much...and so she is more comfortable about it, and trying to reach out to you.
And there's the matter of -- it's her wedding. The biggest and most important thing that will happen to her...at least right now. She's consumed with all the planning, all she can talk about is all the planning, all the details, getting it "right"....just as some folks are consumed about their diagnosis. So that is all she can talk about right now.
Whether you want to participate in it with her, and see if you both can get past that, is entirely up to you. But that's my take on it.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....