I don't think anyone can give you a better answer than Nature's own website. Obviously the text there is vague, but that should be expected. It's basically marketing material. Nature started these series presumably because they could (and probably to snatch up market-share). Publishing is a business after all.
With that in mind, I think you are lumping editor and publisher goals together. Nature cares that each journal is successful - not necessarily how it fits into the portfolio in terms of "ranking". So what does that mean? To a publisher, success likely means that a journal makes money. Impact factors, prestige, scientific rigor, and all those other things that authors and editors care about are somewhat secondary to this - I think it's a bit naïve to think otherwise. So, there are probably 5- and 10-year plans but on the publisher side I would expect them to be business-focused. And on the editor side they will be too journal-specific to make a statement about the series as a whole.
The most obvious way to get an idea of "ranking" has already been mentioned: impact factor and other published metrics. Since these aren't available for every single journal yet, you have to be a bit more holistic. I think, if you are considering submission to one or another journal, you should take the time to read the journal website. They will all include an aims & scope section. If you want to see what the editors are actually accepting, just read some articles and judge for yourself. Their expectations will be reflected in what they choose to publish. I would expect each and every journal to be slightly different in that regard.
Unfortunately I don't think there is a quick and dirty way (other than IF) to compare all of the journals in those series - you would have to go through each journal yourself. And each journal is likely different enough that you won't be able to make an accurate general statement. As for long term expectations, just assume that neither Nature nor the individual editors what to run a journal into the ground.