How do i tell my sister she isn’t a bridesmaid?

I agree with PP, you don't need to explicitly say anything. If asked, just say, we just decided to keep it to a small group of closest friends.

You don't need to proactively tell her, that could come off as spiteful, which I know would NOT be your intent, but if your relationship is already strained it would only add fuel to the fire. If she asks, you can have that conversation at the time.

My fiance and I arent including family either. On his side because we dont want a couple of them, on my side because I have too many siblings and we wouldnt be able to include any friends. I just told them that we preferred giving our friends that place of honor otherwise we would have parties that were at least 10 people each and we'd rather keep it smaller. We're essentially using my family as the scape goats because my siblings are chill about it. If your sister asks just say youre not including your fiances siblings so youre going to keep it consistent and not choose any family to keep numbers even.

I wouldnt necessarily mention anything to her outright, this could also be your mom just projecting her wishes for you and your sister to get along and your sister may not have actually said anything.