Brother of the bride!
Historically, there isn't a lot of glamour to this role... (no movies by this name that I can think of...) but despite the lack of fanfare, let me tell you – your sister still needs you to show up and look great. In fact, that could be the only thing you really need to get right! Look the part, smile, and throw all the attention towards her. And remember, the toasts may or not be remembered – but those pictures are going to last a loooonnnnnng time. So let's get you started.
Like you, I would be conflicted about wearing business casual to a wedding (even in 2020). You want to look dressed up and put together, but not in a showy way – the "full on suit" option you mention wanting to avoid. My advice would be to go for a blazer, dress trousers and a quiet tie. You can always lose the tie and undo your top button if you are feeling too stiff – but I think you'll feel better and more confident if you at least start out the day looking worthy of the occasion.
The Carolinas in October are pretty awesome – cool but usually not cold – so it's a great time for a super-fall-feeling sport jacket. You probably don't want to go too professorial with anything particularly tweedy or textured, (unless that's your sister's vibe,) but we've got a lot of greens right now, which could be a cool option for you. The color is festive, and not as expected as navy. I may be thinking this because I brought this hopsack the last time I was in North Carolina — it was for a birthday party, not a wedding, but I still felt pretty good. This one's got a quietly cool herringbone pattern with a bit of extra interest up close, but it's not going to register as anything too wild from a distance... six-foot or otherwise. Think about how good it'll look in the photos with some changing leaves in the background! Underneath, a light blue dress shirt, charcoal dress trousers, and a navy club tie. You can have some fun there if you want - maybe make it a tribute to the couple if they happen to be into squirrels or tennis or vinyl records. Or a classic dot is always a safe play. If a green jacket isn't your thing, just sub it out for another herringbone in oatmeal/chocolate or blue/sage... or the undefeated champion, the navy blazer. Everything else can stay the same (although if you go with solid navy, you can kick up the tie a little more... maybe they like sparrows?)
Lastly, shoes. Dress shoes for a wedding are an obvious layup, but as a wrinkle I might suggest dark brown suede Chelsea boots. (I literally count down the days until it's cool enough to wear them – my personal threshold is 65 degrees.) They're refined, but can go a little 'country' and 'mountain' in the best of ways. When your feet are warm, the rest of you is warm... so make sure you've got a solid pair of socks (navy wool is solid,) and you'll be able to handle any damp grass or chill in the air.
I have to say, Andrew, I loved getting your submission. I know it's not a rocket-science answer – coat, tie, trousers – but the question felt optimistic. There has been a lot of ink spilled about how we are all dressing more casually since our lives have been so affected by COVID... in many ways, almost put on pause. But weddings (and sadly funerals,) birthdays, bar mitzvahs, and anniversaries will continue. The gatherings look different – usually smaller and less public – but celebration and ceremony still ask for effort. These are important events and they're worthy of us stepping it up a little. Dressing up isn't canceled. And that's all you really need to remember. Again, you can always take it down a notch once the party gets going. But start with the tie on, keep your feet warm, and make sure you're ready to dance.