I'll have a look at the sources you mentioned.
My running theory is that Maeve at some point manages to leave the demiplane of dread through the plane of shadow (which is technically possible), and at some point afterwards becomes the QoAaD.
I guess in order for people to challenge this theory i have to list the reasons why.
We all known that when an individual moves from their home plane to the demiplane, time is disrupted. They could spend fifty years in the demiplane and it would be like they never left. We also know that time doesn't work the same in the Shadow Rift , one year outside is a century inside. If Maeve were to manage to leave the Demiplane, time would evidentally not work as it would for others. We've also seen from some crossover lore than on occasion individuals leave the demiplane and sometimes they are flung far in time from when they expected. I believe in this instance, the term sister is used to describe that they are in fact a representatiion of the same being, but opposite. i would go so far as to suggest that the QoAaD IS Titania, which would only be even more supported by the suggested material in Gaelic Folklore that there is really only one Court, and whether it's the Seelie Court or the Unseelie Court, they're all the same fickle beings, just manifesting themselves as two separate entities.
Also, in Gaelic folklore, where a lot of this originated, when there is use of the word sister or brother in relation to fey, it's not always meant by blood or something. Fey don't often procreate normally. Sometimes, Fey simply come into existence from nothing, or simply change from one being into another. Sister or Brother as descriptors are more often used to compare two things than it is to describe a blood relation. I've seen it used in similiar stories to describe something as the opposite of something else, and also the same as each other. It's actually more rare for it to be used normally than it is as i've suggested.
I think the most interesting argument is also that the holy symbol of the Queen of Air and Darkness is a black diamond. You know who also covets a black diamond for it's power? Maeve. And we also know that items from the shadow plane are more often corrupting than they are beneficial.
So, we have a morally questionable fey who uses a black crystal for questionable purposes, is a Queen, lives in a realm consumed by shadow and darkness, and is literally thousands of years old, virtually immortal, by the powers of that black diamond.
There's several more "coincidences" i could mention, but those are pretty hard to ignore.