Wait, why exactly? I can see it being a problem in some cases but I wouldn’t assume that it is generally a problem. The way I see it it is just two people who are loved ones of a common other (which is not uncommon in polyamorous relationships as far as I know) and the power dynamics would be dependent on those relationships, of which I would assume there is no inherently more powerful one.

The acceptable age ranges do imply something icky between the parent and step parent though.

I should also note that I have not had step parents or children at any point in my life so far and may be unaware of otherwise obvious things.