Elven/Mortal marriages...

[Findegil]

Yes, in the case of children of Elladan and Elrohir we can only speculat. But we might es well consider likliness or unlikliness.

We have the following statment about the doom of Earendil and Elwing and their sons (in the published The Silmarillion this was editorialy shortened so that I will qoute from the original Oenta Silmarillion, sub-chapter: The Conclusion of the Quenta Silmarillion in The History of Middle-Earth volume 6: The Lost Road and other writtings, {I added some emphasis.}):

$8. Now the Gods took counsel concerning Earendel, and they summoned Ulmo from the deeps; and when they were gathered together Mandos spoke, saying: 'Now he shall surely die, for he has trodden the forbidden shores.' But Ulmo said. "For this he was born into the world. And say unto me: whether is he Earendel Tuor's son of the line of Hador, or Idril's son Turgon's daughter of the Elven-house of Finwe? Or being half of either kindred, which half shall die?' And Mandos answered: 'Equally was it forbidden to the Noldor that went wilfully into exile to return hither.'
$9. Then Manwe gave judgement and he said: 'To Earendel I remit the ban, and the peril that he took upon himself out of love for the Two Kindreds shall not fall on him; neither shall it fall upon Elwing who entered into peril for love of Earendel: save only in this: they shall not ever walk again among Elves or Men in the Outer Lands. Now all those who have the blood of mortal Men, in whatever part, great or small, are mortal, unless other doom be granted to them; but in this matter the power of doom is given to me. This is my decree: to Earendel and to Elwing and to their sons shall be given leave each to choose freely under which kindred they shall be judged.'
$10. Then Earendel and Elwing were summoned, and this decree was declared to them. But Earendel said to Elwing: 'Choose thou, for now I am weary of the world.' And she chose to be judged among the Firstborn, because of Luthien, and for the sake of Elwing Earendel chose alike, though his heart was rather with the kindred of Men and the people of his father.

If we read with that in mind the doom that was spoken at the birth of Arwen. Then my spekulation is that no further special dooms were ever granted. If one of children of Elrond marriaged a mortal they would choose mortility and their offsprings would be mortal (e.g. Arwen). If they would marriage an elf they would follow thier father into the west. Their offspring in such case are very special. Since they had a small part of mortal blod. But there is no example given.

My speculation is, that the brother did not marry in Middle-Earth because they feared that their offsprings would be mortal what so ever wife they toke. But if they marriaged later in Aman there could be little doubt that their offspring would be granted elven lives.

Respectfully
Findegil