Marriage between Elves and Men

[Alchisiel]

As to the fate of Elladan and Elrohir this might be of some help:

After the War of the Ring, little is known of the brothers' fate. They returned to their father's house at Rivendell, and remained there even after he had passed across the Sea. Like their sister Arwen, the sons of Elrond Half-elven were granted the choice of whether to leave Middle-earth for the Undying Lands, or remain there and become Mortal as Men. Elladan had such a bond with his brother that they must surely have chosen alike, but what choice they made in the end can never now be known.

I found it at the Encyclopedia of Arda website.

Actually after the Fall of Gondolin Idril and Tuor stayed at the Mouths of Sirion for a time until Tuor felt old age come upon him and then they sailed into the West together in the year 560 of the First Age.

Your questions aren't silly there is actually a thread called the Immortality of Arwen that might answer some of your questions.