Celeborn could, of course, have any number of siblings (so long as it's less than six).
The suggestion in Friday's session was 'let's give Celeborn a sibling in Ossiriand, as we are in need of a named character among the Green Elves there.' It could be a brother or a sister, it was just important that this person *not* be in Doriath (with Celeborn). The suggestion to make it a sister is because (obviously) the number of named male elf characters is *significantly* higher than the number of named female elf characters. So, if you don't need someone to be male...maybe don't go there at this point.
While it's not impossible to add more elves to Doriath, we don't have a specific role for an additional Doriath elf at this time. The death of Thingol, Celeborn's king and mentor who saved his life when he was a teenager and he'd followed ever since, is likely a strong enough reason for any of Celeborn's anti-dwarf sentiments. [cross-posted with cellardur]
I agree that the death of *just* extras is pretty much meaningless on screen (here's looking at you, Hela in
Thor: Ragnarok), so you have to establish some rapport with the audience if you want them to care. But...that isn't an issue of it simply being a named character. The Warriors Three die when Hela invades Asgard, and they were in the two prior films. The audience should know their names and know some things about them. At least one of them has dialogue and a singular death scene (rather than being part of a general massacre). But...they aren't reintroduced in the third movie at all, so the audience doesn't even notice that it's a named character who is dying. To do this well, you show that character (even if just an extra), doing normal day-to-day stuff earlier in the episode, letting the audience get comfortable with them, and
then you show evidence that they have died/their death scene. I haven't watched
Schindler's List, but I know about the little girl in red, because that was so powerfully done that people talk about it.
So, here is the liquidation of the ghetto scene:
It's a black and white film, so the only color on screen is this young girl's red coat. You see her move through the scene, seemingly immune to what is happening around her. The onlooker (and thus the audience), is clearly rooting for her. You lose sight of her and focus on other things, but keep coming back to her, until finally the end of the scene is from her point of view alone. You see people murdered on screen during this scene, and you see the onlooker being horrified and wanting to look away. And yet...the focus is on the little girl in red.
Which makes it all the more powerfully tragic when this happens:
You don't need to know her name or anything about her for that sequence to be powerful. And it's certainly a million times better than showing the deaths of nameless faceless people and expecting the audience to feel the horror of it.
If we find we are coming up on a massacre and haven't introduced any named characters to die in it, that's when you spend a moment getting to know the common folk and seeing things from their perspective. Peter Jackson's
Two Towers showed us two children fleeing from the Uruk-hai sacking their village, with the fate of their mother unknown. Obviously none of those characters were 'important' to the story, even if they did have a role in the plot. But part of the point of telling the story that way was to make the audience feel the threat of war better than simply showing an advancing army would do. You are supposed to worry when the little girl pipes up and asks 'Where's Mama?'