Most likely maternal or grandmaternal. No, I don't have any notion of who might form an intermediate generation, don't ask.
If Tzardok is interested in Eldancanon, or my take on things...

The Queen of Air and Darkness and Titania were sisters, when the QoAaD was corrupted by the black gem. That much is more or less certain.

In my personal headcanon, they were both daughters of a high King of Faerie. He's unnamed, though sometimes I've called him "Balor" in my head, though that name is sadly taken in D&D. Back then, there was a Plane of Faerie, but the war between the two princesses over the succession and the Black Diamond sundered the entire plane, into parts that are now the Seelie and Unseelie Realm and - and this is a lot of headcanon right here - parts of the realms of Dream, Shadow and Mirrors and a seed of the Infinite Staircase. Rhiannon, in that headcanon, is the originator of the fey, Titania's grandmother, who now takes the role of grumbly, old and immensely powerful crone who advises the queen.