Ring bearers mother!

It honestly sounds like she's trying to force you to remove your nephew so she doesn't have to pay for any suit, rented or bought. You need to put your foot down and remind her how it's your wedding, not hers, and she doesn't actually get to dictate what your wedding party wears. Give her the choice of either getting your nephew fitted or removing him from the party, herself. How she cooperates with your request is entirely up to her. You've done your part.

I agree with @found my price. Its YOUR wedding day. Does anyone in the family see them often? I would have someone go over there and get his measurements and order the suit yourself. Or figure out what size he regularly wears and order that or maybe a size up to make sure it fits and then when it comes in have him put it on and see if there's any alterations that need to be made. It's hard when it comes to people in the BP being difficult but she's known this has been coming for awhile so if she had that big of an issue with it, she should have spoken long before now.

Sis needs to back down and realize that she agreed to this when you asked her to allow her son to be the ring bearer. She knew what she signed up for when she said yes to be your MOH. Honestly after that much bs she wouldnt be my MOH. thankfully my moh doesnt have kids yet lol

My son is my ring bearer and my FH and groomsmen are buying their suits instead of renting because it is makes so much more sense. Its a difference of about $30.. at any rate I'm buying my son a SIMILAR suit in the same color because it's cheaper to do it that way and again just makes so much more sense... OP I understand that you want things to LOOK a certain way, but this, to me is one of those things that's not worth making a big deal out of. . Let her go to Macy's or wherever to buy the similar suit and move on. I promise it will not detract from your day in the least!

OP you're getting a lot of different opinions and I'm sure that's confusing so let's sum this up...

At the end of the day it is your day. But do you want to lost a nephew and best friend over a suit? It truly won't matter that his is different, and as PP's have said his difference won't be noticed.

That being said it is your day, and people in the bridal party really have two jobs: show up in the outfit you choose relatively sober. Since the second won't be an issue for your nephew, it will be the outfit. If you want it, that's what you say.

At the end of the day it's your choice and no one's going to judge you for what you choose. Either choice is valid- it just depends on what you and FH want. I'd talk to him and see what he thinks too. It's ultimately your decision. Good luck, and I'd stop listening to other opinions and decide what you're going to do!