Simply put there is no such thing as a “stepsibling” in French kinship terms. More accurately, if demi-soeur/frère is used that way (like we use beau-père/belle-mère for both stepparents and in-laws), I do not know of it.
I’m sure people who HAVE stepsiblings (it just so happen that I only have a half-sister and do not know people with stepsiblings) have strategies to discuss it. Myself I usually say I have two sisters, THEN specify one is a half-sister, since I consider both my sisters (from an interpersonal point of view, not a genealogical one).
I use to translate stepfamilly by famille par alliance, so I think you could use something like une soeur par alliance to translate stepsister.
Most people who share affection with their step-sibling will consider them as their demi-frère/soeur and use this term, myself included. However, the problem lies in the bounds within the family.
half-sister : demi-soeur
stepsister : belle-soeur