Sisters of Twilight
A long time ago, in Athel Loren (the Black Forest, located in scenic Alsace-Lorraine), there lived a very young Elf girl named Naestrahan. She was lured away from her parents by dancing lights and whispers of forest spirits, leading her so far into the woods that she reached one of the places even Elves and the Treekin are afraid to enter. Nobody knows what happened (they aren't telling), but some time later two girls, identical to Naestrahan except one had black hair while the other platinum blonde (or white), returned to the Elves. Her essence had been split in two with the resulting women Naestra and Arahan becoming the masters and if need be the judges and executioners of the fauna within the forest.
Their personalities, as the trope demands, are mirrors of each other (sweet/sour dusk/dawn (vampire-less) twilight theme obviously). Naestra is a kindly friend to all beings, a veritable Disney princess in the grimbrightness (or nobledarkness?) of Warhammer Fantasy. Any creature aligned with nature serves her willingly in any capacity she requires from giving themselves for food or materials to becoming an army of deer, bunnies, and small birds. This is much more badass than it sounds as anyone who has hit a deer, been bitten by a rodent, or seen The Birds can tell you.
Arahan is the opposite of her sister (or self, whatever you wanna call it). Arahan is only seen sadistically gleeful during battle, or scowling and predatory in times of peace. Arahan commands the wolves, birds of prey, and bears of the forest to battle alongside her as the de facto alpha female of all of nature.
They became immortal beings of legend, living on the mountain tops in the middle of the forest and appearing anywhere they are needed without needing to be summoned. The two have a neat little trick they use in battle; if one dies while the other survives, they both pop back to life. This isn't just a crunch trick either, as in the End Times: Khaine event Naestra sullenly chews out a gleeful Arahan after a battle, immediately before another, for "finding new ways to get herself killed".
The two commonly ride a Giant Eagle named Gwindalor, or a Forest Dragon named Ceithin-Har (it isn't clear if he has the level of intelligence as the Dragon of the Asur).
If you're having trouble telling them apart, they both wear half-masks like most Wood Elves. Naestra wears hers on the right side (her left), Arahan wears hers on the left (her right). Also to contrast their dusk/dawn theme, the nice twin (Naestra) is the black-haired one in the picture to the right while the vicious twin (Arahan) is the white-haired one.
Unfortunately, they apparently finally perished together in the old Warhammer setting as they didn't appear in Age of Sigmar among the other characters who became gods and their model was dumped along with many others into the Last Chance section before being removed from the site.
On The Tabletop
Sisters of Twilight are arguably one of the best options the Wood Elves have. Toughness 4, can ride either a Giant Eagle or a fucking Dragon (both have their merits), and unless both are killed in the same turn they both return back to life at full Wounds at the end of the Shooting Phase. Naestra shoots like a fucking bolt thrower with a bonus of healing their mount when she shoots something and Arahan's machinegun style shots are Poisoned. Naestra deals more damage to Destruction enemies, Arahan deals more to Order enemies (no bonus against Neutral). This is on top of the usual statline for Elves including Leadership 9, Always Strikes First, Initiative 7, Ballistic Skill/Weapon Skill 6.
Think the above isn't enough to justify taking them? Here's what happens when you take them on their Eagle. You stick them near Warhawk Riders for a 4+ Look Out Sir! roll. Still not enough? Cannons can't target them and what is hit in a multi-model profile is randomized, so if only one of the two twins was blown to pink mist all over her sister then they'll shrug it off as she magically reforms the second the enemy stops shooting, making it a total waste of cannonball. On the other hand you can make them sniper bait and pay through the teeth for the Dragon, and what you get is a fucking Dragon that can and will eat Units and regain Wounds like a vampire when Naestra fwips something.
The only real drawback is that if their mount dies, they have to stay together. But that's not exactly a deal breaker to say the least.
Taking them on Gwindalor (the Eagle) will set you back 275 points, something very doable for a great option. Ceithin-Har brings them up to 495, the same amount that High Elves pay for Tyrion (needless to say, you'd be getting the MUCH better deal). Due to the prevelance of artillery (especially cannons) in the game, Gwin is generally the safer option due to survivability but Ceith is the biggest bruiser available. The only problem is that the kit comes only with Ceithin-Har, meaning you'll need to obtain a model for Gwindalor separately. Most players looked to the superior sculpt from The Hobbit for the Giant Eagles and simply plop the two sisters on its back.
In Age of Sigmar, many MANY models and entire races were Sqatted including the model for the Sisters of Twilight which sold out very quickly once they were moved to the Last Chance section. Fortunately, its extremely easy to obtain (*Cough* Reaper Bones *Cough*) Dragon models to put two Elf women carrying bows on, with a small greenstuff halfmask on each if you're so inclined.