Sister (moh) baby during ceremony?

He doesn't need a role in the ceremony, but if you're really determined maybe you can get a wagon that someone else can pull? He can be in the wagon. Otherwise, I think it's totally fine to just have him be in the photos afterward.

I think it would be very sweet for you to include him but personally I would caution you against it. He’s only going to be a few weeks old and having heard how newborn babies can screech and cry loud enough to wake the dead, I personally wouldn’t risk it unless your sister does not feel comfortable leaving him with a babysitter for a prolonged period of time.

Don't put any performance
expectations on an infant. It will choose someone's arms, it's stroller , or someone else's arms as a safe and quiet place. And fuss or cry anywhere else. So go for comfort and quiet.

I wouldn't specify a role for a 1 month old honestly, just wait and include him in pictures

At one month old, there's not much you can really do. It is a sweet gesture though. Maybe find him one of those adorable tuxedo onesies?

Thanks everyone. Maybe I’ll find a cute baby onesie and that’ll be my contribution

Yes, I agree. A cute onesie maybe and some photos. But BE CAREFUL he doesn't spit up on your dress! It would certainly be a funny story to tell at HIS wedding in 25+ years, but it will be likely make you quite upset day of.