How to tell someone their partner is not invited?

This is a long one so please bare with. My older brother has severe autism and requires someone with him at all times. He lives with my ‘step dad’ who I don’t really get along with but as my mum passed away a few years ago I’ve had to deal with him. Anyway he has a new fiancé and I don’t like her. My ‘step dad’ is only invited as my brothers plus one but a few months ago we had a conversation and they implied my ‘step dads’ fiancé was invited. Nowhere on the save the dates did it say her name. It actually said my brothers name first followed by my ‘step dads’. We are sending the invites out in a few months and once again I will be putting the names in that order. How do I bring up the fact she’s not invited without causing conflict? My brother means everything to me and I need him there so I’m worried he’ll turn round and refuse to come. There’s no question about inviting her by the way, it’s bad enough I don’t have my mum there but there’s been conflict with this woman in the past and I don’t want to see her face 😂