I don't think the original rules would be equipped to handle modern society with our different types of families, people have children without getting married, the frequency of divorce and remarrying, etc.

Perhaps, you could author a new guide for us all. You could probably just make up some rules, because I am not sure anyone really has the answers. Or, you could check the latest versions of Emily Post or whatever source for proper manners. There would likely be an explanation to be found there.

However, in every day life, I just find it easier to use the word that best describes the relationship and don't worry about what is technically proper. I believe if someone is asking another to use a definitive term, they are most likely simply being nosy about the situations within another's family. And, for me, the most important question is simply how do we relate as a family and what do we call each other.

I have always referred to my grandmother's sisters and even her aunt as simply Aunt So-and-so.

My sister never married her boyfriend, although they had 2 children and lived together for 15 years. I usually refer to him as my brother-in-law, even though there were never any legal connections.

My mother dated a man for years and they never married, but I became close with some of his children from his previous marriages. In conversation, I would refer to them as step-siblings, however we are not at all related by blood or by marriage.

I have many people that I call aunt or uncle or cousin and simply skip the second, half-, step-, twice-removed nonsense. If someone is that interested, they can ask for a family history. Those that do are typically half-asleep by the time I get through my grandmother's aunt was his step-mother, but we just call him cousin, even though he recently passed and was quite a bit older than my own mother. To make matters more confusing, I became friends with his step-daughter. Now, how in the world should I refer to her? I jokingly say that she is my "second-half-step-cousin-twice-removed." I have no idea if this even comes close to the proper terminology.

As for your situation, I would only use "step" once, and only when pressed, and only choose either great or step, not both. If you are the aunt, they probably call you Aunt Duckie or whatever. And, I see no harm in buying them a birthday card for niece or nephew. I'm pretty sure that Hallmark doesn't have cards ready for all of the many possible relationships that exist these days.