There is so much we don't know here that a good answer is impossible.  What kind of business structure?  What are you producing?  On and on.  However, let me toss this your way.

The first thing you have to do is forget the 2.0 part of things.  She is your brother's wife.  Period.  Yes, previous  entanglements have an affect, but they are not relevant.  This is how it is right here right now.

How does your wife feel about it?  Is she involved in the business?   How does she get along with your sister-in-law? Perhaps you need to ask her ideas on how to make this better?

You have received some good advice.  I recall reading in a book someplace to be kind to your enemies and in so doing heap burning coals on their head.  Or in other words, there are only 2 ways to get rid of an enemy.  Kill them or make them your friend.  Maybe you need to find some way to get the SIL involved in the business?  Or maybe splitting the business is the solution.  Either way she is still your SIL.