Will try and avoid drip feeding, but also keep this to the salient points so fingers crossed I give the right/enough info.
Exh is in a new relationship (well, 2.5ish years, but newer than ours as it were), has got engaged to his girlfriend, she is still married to her exh as he won't agree a divorce.
Exh's fiance has two children and my DD gets on well with them, esp her DD.
Here's the AIBU - DD has been referring to this girl as her step sister. When I gently pointed out that daddy and X aren't married yet, so she will be your step sister, but isn't yet, DD got huffy and said that daddy, X and X's daughter all said they were stepsisters. I repeated that they're future stepsisters, but that X needs to get divorced and married to daddy before they're actually stepsisters.
Other potentially relevant facts:
DD is 6 and can be very dogged about things she thinks are true. DS, who's older, has ADHD and ASD traits and gets very literal about things, so I don't want to set DD up for spouting facts contrary to the truth as DS will argue and neither of them will it go.
However I am also just pissed off that exh and his new family are, essentially lying to my daughter, so I admit I am not 100% sure ianbu because I admit that there are emotions in this.
YABU - let them say stepsister. It'll happen one day and why not let them use the term now?
YANBU - stick to the facts and keep correcting her gently by saying future stepsister - you can't even be sure they'll get married until it happens.