Okay, my bad again for forgetting to mention that - but I did place a single hell totem to buff their levels to 50.
From what I've seen the native/large females are around level 60+ from that for some reason.
So, once again, 59 raider count per raid, 10 Halloween raids, level 60+ raiders = 0 elder sisters.
Well as you said then, probably some other unlock condition needed then. This game is filled with random unlocks the game doesn't warn you about, especially for crafting shit. At least if you're at that point in the game, getting an elder sister naturally to test it shouldn't be too difficult. The most annoying part for me is the Gold Fish since fishing in this game can't be improved and is slow as hell. At least Gold Fish are common, I originally thought it required Golden Fish.
Though now I'm going to test the elder brothers as well, I never made one naturally. Maybe that's why they won't show up if they can be summoned in raids.
Edit: Nah, doesn't seem like elder brothers show up in Halloween raids after you make one naturally. Reverting to old save since I don't want an elder brother anyways, lol. Wish they didn't lose their old crafting recipes.