The six children of Elrond Earendilion and Celebrian Celeborniel were Elrohir, Elladan, Andreth, Belemir, Arwen, and Aragorn. Three would reunite in Valinor, three in the Halls of Mandos. Celebrian had never met Aragorn, but Elrond was sure she would have approved.
Elrond married Celebrian when he was finally able to conquer his fear of risking loss. Celebrian understood the delay, and forgave Elrond for it, in time. Though not all of her adopted brothers did.
They had children a mere century or so after their wedding, because Celebrian didn't want to wait any longer. She loved elflings, and wanted a large family.
The twins were born first, and almost last. Well, Elrond ran out of courage, after the twins. And he was terrified of a daughter who might take after Galadriel. Terrified in a very warrior-like, brave way, but terrified.
But Celebrian wanted more children. And the twins had grown into brave, mostly responsible, young soldiers, who were as often Glorfindel's lookout as Elrond's. And Melpomaen, the adopted son of Elrond's gwador Erestor, was a calming influence on the twins.
So Andreth was born. If Galadriel was starlight, Andreth was sunlight. Sweet and open and warm, beautiful and kind. Elrond was enchanted. The twins were, as well. Celebrian was delighted to have a daughter, though she made sure to have time for her sons, for Elladan's experiments and Elrohir's strategy games.
Then, before Andreth came of age, when she was a charming if willful teenaged elfling, Belemir was born. Elrond was confident; he had handled two sons at once, surely he could manage this one? And at first, Belemir proved less mischievous than the twins had been. But Belemir was still a very young elfling when Arwen, their surprise, came along.
Amongst their family and closest friends, the children of Lord Elrond and Lady Celebrian were thought of like this: there were the twins, then calm Andreth in the middle, then the "little twins," Belemir and Arwen, who were less than a decade apart in age.
By the time Lord Elrond took Aragorn, son of Arathorn, as his foster-son, and re-named the boy Estel, only three of his children remained in Middle Earth. Belemir had made Luthien's choice, and wed a distant niece of Elrond's, a daughter of Firiel's and Arvedui's. Then gentle, scholarly Belemir had died young, defending the last brave remnants of Arthedain, of Arnor, from the Witch-King of Angmar and his armies. Then Celebrian and Andreth had been captured by orcs, and the twins and Melpomaen had been in time to rescue Celebrian, but not Andreth. Elrond could at least comfort himself that he would see Andreth again someday in the West, for she had wed an elf, who chose to keep Celebrian company on her voyage.
It did not entirely surprise Lord Elrond, when Arwen fell in love with his last "child," his human foster-son Estel. Arwen, like Belemir, had always had something of Elros about her. A certain lack of hesitation, when opportunity came along. For Arwen, the opportunity was a short lifetime spent with the man she loved, one of the best men Elrond had ever known. And elves, as a rule, loved but once. Elrond would miss his youngest daughter forever, once he had sailed. But he would not begrudge her Aragorn's love, nor Estel Arwen's.
What choice the twins would make, human or elf, Lord Elrond could not predict. He had never been able to predict the twins, though he loved them dearly, and hoped to see them again someday, in the West.