@Imissmoominmama
I can’t speak as a step mum, but I have one biological child and two adopted. My love for all three is immense.
I’m not entirely convinced it’s about biology. It’s more about responsibility (and to some degree authority).
The key difference between your own children (however they came to be your children) and stepchildren is the level and kinds of responsibility involved. I do not, and cannot, have the same responsibility towards my DSC as my DC. They have their own parents who are responsible for them. One of the many things this means is that I have no choice or control over large aspects of their lives, and even the areas I might have any input are mediated by their father (who retains both responsibility and authority in relation to them). That inevitably affects how I feel about them.
With my own DC I can make decisions (even if I share that authority with their fathers) but crucially I know that I am always responsible for them. They need me in ways the DSC never will, and parental love is one of the ways that nature has ensured my DC get what they need from me.
In adoption you take on that same parental responsibility (in both the real world and legal senses) and build a relationship on that basis. As a step parent, you are always at some remove (there will be different degrees depending on a range of factors) and your relationship will reflect this.
It seems absolutely logical that this will translate into many of the feelings described in this thread: the kind of fierce love that would have you die for your children vs the various different kinds of love described by steparents (akin to a nephew/niece in some cases, or more limited in others).
Personally, I find it hard to love my stepchildren. Truthfully, I don’t love them and I don’t think I ever will. That’s because of a whole host if things to do with their parents, and DH in particular, that don’t look like they’re going to change. I am kind and caring towards them and I try to ensure I treat them fairly but I have to disengage and step back for my own sanity (and to make sure I fulfil my responsibilities towards my own children). Even though he loves them very much, DH currently struggles to like his own children at the moment (but won’t engage in the kind of parenting that might make their arrival something to look forward to rather than dread, for several reasons - some more sympathetic than others). Given that, I think managing to maintain kind, respectful but mostly neutral (how I’d treat a child if I encountered them professionally) is doing very well indeed. Asking me to love them like my own children would just be a cruel joke.
Under different circumstances I’m sure I might love them in the way I love my nephews, but the circumstances are what they are and I can only do my best with what I’ve got.