She is not the queen of the void. Rek'Sai is the queen of a voidling-race in the Shurima-Desert. These creatures, even if they are not real animals, still do have a lot in common with insects. Most insects don't have a king, and all that aside, a king wouldn't fit Rek'Sai at all. Since all Rek'Sai is, is to be the strongest of her kind. The average Xer'Sai is way smaller than her, practivally yordle-sized. She is special. If you would create another Xer'Sai, just as special as her, what would she be?
Lee Sin comment completely emotionless written. Just as there is since the Lore-Retcon no Institute of War anymore, there are no summoners. They got banned from further lore. They just don't exist anymore, and soon they wouldn't even ever have existed in Runeterra.
If a champion has buffs, which only occur if other champions come in, it is called a hidden passive. Sion gets 1 more gold from killing Zyra vice-versa. Leona deals 1 less damage to enemies with sunglasses with her passive. If Caitlyn or Vi have Jinx in the enemy team they get a kill-counter. After 30 minutes ingame, when both reached level 16, Rengar and Kha'Zix will start the hunt, if they are in opposing teams.
Thats what I mean. If your Champion has significant buffs with his sibling, he would close to only be played while the other is in team. To balance this, he would either be weaker than other champions alone, and as strong as they are with his sibling. Or he would be as strong as every other champion, but stronger with his sibling. This is, after all, game breaking. In first case, he would force other players to play his counterpart, or be flamed cause he is not as strong alone. In second case, he would be stronger than other champions, making it unfair.
BUT if your champion only synergizes good with his bro, that would be ok, but we already have that. Kalista/Lucian - Thresh, Zilean (with new passive) and Mordekaiser can pull some crazy sht, but if you would have to rely on the other champion for certain parts of your own, that would be simply bad. The best example for this is Kalista:
If your Soul Mate is AFK, you can't use your Ult and you can't proc your W.
As I said before about Lee Sin:
A while ago Riot Games decided to delete the Institute of War (the League of Legends) from the Lore. Though, some lores still mention it, it won't come back and won't be mentioned anymore. In future lores the League simply never existed.
-> So, how destroy something that never existed in the first place?
I play League of Legends since 2011 and yes, I am still playing it, and yes, I like the lore and yes, "I love this game". I didn't quit the game. Without those mysteries, what would it be? I don't want to know everything. Why is Amumu cursed? Why does Rammus exist? Where does Jax come from? Why is nobody allowed to touch TF's hat?!?!
These are things we either find out in Events, like maybe Harrowing, or such as the Burning Tides, where was plenty of lore about TF, Graves, Gankplank and MF.