Young looking even for an Elf, Nemiril is Second of Amon Ereb that guards the corridor from Doriath to Lindon. Her family was broken in the First Battle with her parents being killed but her brother becoming one of the most vehement of the Guest Elves in his hatred of all things Noldorin. To him, they brought great evil in their wake and he will have naught to do with any who would deal with them. The fact that that was impossible in the world they now lived in made no difference to him.
So it is that Nemiril and her brother are kin no more per his attempt to force a choice upon her that she would not make. Their mother, Dagriel, had been a handmaiden to the Lady Melian before she wed and they had many aunts, uncles and cousins among the folk of Doriath. The knife's blade the King walked was hard enough -- he was many things but no fool. Limit contact he could. Prevent it completely as Harnon wanted to do?