I think that showing Earwen grieving the death of Olwë will be important. It might be a little tricky to pull off in a limited amount of screentime, but showing Nerdanel refuse to join Fëanor is probably the 'big deal' Valinor-war-widow. So they get a farewell scene. We could show Anairë and Earwen standing together and refusing to join Fingolfin and Finarfin in a silent scene that is a bit of the 'Noldor leave Tirion' montage. Then, in the aftermath of the Kinslaying, some of the elves come down from Tirion to Alqualondë and just stare in horror at what was done, and that's when we see Earwen grieving for her father and Finarfin being stricken with second thoughts about following his half-brother any further. In other words, we can include this detail without having Earwen originally decide to join in the rebellion. If we want a fuller story, she could go from Tirion to Alqualondë to warn Olwë of the Noldor's rebellion, only to be on hand when Fëanor arrives and her father is killed.
It's 'important' because we have to remind the audience that Olwë was family to the House of Finwë. His daughter is Fëanor's (half-)sister-in-law. The easiest and most poignant way to show that family connection is to show her grief at his death. It is not for nothing that this is called a 'kinslaying'. Yes, on one level, that just means 'elves killing elves'. But they are actually kin, too.
I realize I can get carried away with wanting all the details of the story in an adaptation. We don't need everything that Tolkien ever wrote to tell the story of the fall of the Noldor well. But at least considering the different aspects and then dismissing the parts that don't help us seems like a better idea than leaving them out without consideration.
I think that making Fëanor's half-sister Írimë a hot-blooded supporter, backing up everything Fingolfin and Fingon are saying with a 'yeah - let's avenge dad's death!' streak will help to show that Fëanor is not alone in his grief turning to anger and a desire for vengeance. If that over-developed sense of vengeance gets her into trouble when they steal the ships....well then, that's some quick karma and explains why she's not really in the story. It also might explain why the host of Fingolfin, coming up to find the battle already in progress, so quickly join in on the Kinslaying. If Fingolfin's sister is killed as soon as she arrives....well, I would think that would be an opportunity for some rash actions all around. We can work with that.
We would have to introduce child-Írimë in the same episode with young Fingolfin and Finarfin (ie, last episode), but I think her role in this episode and moving forward would be to take Fingolfin's side in all of the Fingolfin-vs-Fëanor feuding. She would be a supporting character who shows up in a few scenes. Which might make it all the more surprising that she's as hot-blooded as Fëanor (albeit less crazy) when the decision to go to Middle Earth comes up.
Keep in mind that Fëanor's sword-training is something he will want to keep a secret from his half-siblings, and that will be in later episodes.
