Here are some hints:
Yes, she feels threatened and jealous too!
You see, your new girlfriend is one more person in your life and one more female in the clan!
From now on, your business partner's wife will have to compete with her for attention and social position at parties, events and in whatever interaction you have.
I know this can sound strange but people are often jealous of other people's happiness.
They feel threatened because it tells them that they could be doing better with their own couple.
It's a competitive thing.
This couple knows you well. They know that this new woman might have a say in the future business decisions you make.
In other terms it's a new player in the game.
They are testing you and testing her right now.
You see, if it's just you, when important decisions need to be taken, a couple against an individual gives them an edge and maybe even more power in the balance.
If it's couple against couple, that's another story, especially if your new girlfriend has her own opinion on these decisions.
The power that was shared between 3 persons is now shared between 4.
Mathematically, it's less for the first 3 players.
I know that your new girlfriend is not that far in being involved in your business but they can foresee a new input of energy and opinion.
That's why they have this emotional reaction.
They know that eventually, if you stay together, she will be a VERY influential player in your life.
The fact that you date someone new means they have to readapt and this challenges them.
Their emotional reaction is the sign that they feel destabilized by that.
Now, it doesn't justify the harsh judgments they projects on you.
They are adults! they should know better!