Have you ever had your children cross it?
My DD is going through some issues at her dad house right now and I think part of it may be her mothering of her half-sisters. DD1 does the same thing when she is home. (My kids are 18 and 21 and their sisters are 8 and 2). She finds being there very frustrating because she has very set views on what is right and what is wrong with taking care of kids (i.e. showering more than once a week, not giving them cookies before bed, bed times - things that she has had but her siblings have different), and I'm trying to remind her that they are her sisters and not her children.
The further issue (and this is really none of my business, but for advice purposes): when she is there or they are out for dinner she is the one who says 'no' or tells them to shower or go to bed or to stop hitting each other. No one ever stops her or says anything such as 'these are your sisters, not your kids. Please stop directing what they should/shouldn't do,' but they become irritated that she is acting more like a parent than a sister.
How do I get her to note her actions and step back?
Well....I don't see anything wrong with what she's doing, if it really is that black-and-white (i.e., she's only giving them instructions that are common-sense and in their best interest).
You say "DD1 (the 21-y-o?) does the same things when she is home." To the 18-y-o? I'm guessing the 18-y-o can diffuse that on her own. If not, talk to DD1: She feels the need to do a certain amount of mothering for her 1/2-sibs at Dad's because she feels the adults their are not doing their jobs. Then, when she comes home and tries to mother your younger daughter, it communicates that you aren't doing your job. Is that what she intends? Probably not. It's probably just a knee-jerk response, after she's gotten used to doing it at Dad's. But now that she knows it seems disrespectful to you, you expect it to stop. Besides, it's natural for an 8- and a 2-year-old to grow up feeling like a 21-y-o sister is a 2nd mother (half-, step- or otherwise!). But her 18-y-o sister should be able to feel like she's a friend. Warn her not to mess that up, by trying to be her mother.