Awww, thank you for asking! Spoilers under the cut:

I have named her ‘Aelir’ (”Birdsong”) and she was a Silvan Elf, not a Sinda. Her hair was very pale gold, rather than the Sindar white/silver. 

She was incredibly close to the trees, even more than is usual for Elves. She would have been the sort of Elf who began ‘waking up the Trees, teaching them to speak’ and walking with the Ents, had she been around in the Age of the Lamps, for example. 

She was tall and athletic, and never wore gowns or jewels except for a single necklace of white gems, a courting gift to her from her husband. She always wore green, and usually went barefoot, with grass-stains on her feet. Thranduil courted her for decades, fascinated by her strength and her freedom and her wisdom and her gentleness, all the joy she found in her home and in the things that grow. She was not a skilled and deadly warrior, as he was – but she was quite a wild thing nevertheless, forever clambering into the canopy of the trees like a squirrel, or flitting through the forest, constantly singing to the leaves and the sky. She hated the idea of being confined to formality and pomp (not unlike Bomfris, but of course Bomfris wouldn’t give two hoots about trees) – but in those days Thranduil was gentler and less chilly, and his tenderness and respect eventually won her over. She carefully unearthed his deep-buried heart as though it was one of her beloved trees, and she coaxed it back into bloom. They would dance amidst the leaves season after season, lost in the whispering of the wind. 

She loved completely, and fully – her husband, her children, her home. Her elder two boys were much like Thranduil in demeanor, dignified and reserved (though Laerophen gained her lanky height, and Laindawar had her lithe, squirrelish strength and her delight in the woods). However, her third child was most like her in spirit – in wide-eyed love with the world, singing constantly and heedlessly, sensitive to the green, slow unspoken world of growing things. (ALSO he managed to inherit her slightly obvious and oblivious manner!!)

As Thranduil became more and more involved in trying to keep his Kingdom free from the taint of the growing darkness, he missed the first signs in his wild Silvan wife. By the middle of the Third Age, Aelir had sickened greatly, practically reflecting the sickness creeping through the wood, and it was too late for any healer to halt its progression. The only cure was to go over the sea, to Valinor, where healing would come. 

But once there, there is no coming back. No ship comes East through the mists.

It was an awful time. For all of them. And yet another loss for Thranduil to endure, surrounding his heart in yet another layer of ice. All he has left, he clings to all the more tightly.