I don't get the feeling Astrid is a jealous one but still I don't think she will be part of any harem, probably not because of jealousy but because of her self respect.
I actually see it differently.
I'd say her jealousy is sprinkled here and there in not so obvious areas;
- When MC first meets her after Astrja's introduction (assuming on WW's path), soon enough she tries to annoy Astrja with the fact that that her and MC can speak English and her sister can't.
- Early on in the game, one of the dialogue choices iirc with her is she questions to MC "I'm much better than Astrja, right?"
Saying yes to her appeasing her pride/ego results in +1 Affinity point with her, while saying the other option does nothing.
- In that book writing scene where Astrja interrupts, there are 2 choices.
One is the part where you choose who to sit next to. Choosing Astrid (despite Astrja still dragging MC to her side) becomes part of action which leads to +1 Affinity point with her, whereas choosing Astrja does nothing.
Not to mention the last choice; telling Astrja "You're getting in the way of Astrid and I focusing on the book" will become part of action that leads to +1 Affinity point with her.
Sure, part of this some may chalk it up to her immaturity given she's younger than other LIs, but even so I'd say jealousy still plays a bit of a factor with her character (even though it's not as obvious as Mathilda).
Characters like Yrsa (who has a daughter) or Candrid (who finally get the leadership that she wanted) wouldn't cross the portal IMO.
And several of the girls don't like the idea of polygamy. You can see Yrsa's reaction when she catches the MC with Yulla.
Compared to Sandstorm, all the main LIs of Snowstorm (Astrja, Nox, Candrid) are in important positions of the heirarchy so I doubt they'd abandon their position in their timeline to cross the portal with the MC.
In fact, I actually think I'm not too keen on the idea of another LI from the second game jumping again with the MC, especially since we've had it once in Sandstorm and it's really gonna come off as repetitive.
Besides, what happened to to the Sandstorm LI who crossed the portal with MC has not even been properly addressed yet so adding another LI who goes through the same thing I'd imagine will just cause more chaos with everything becoming too much of a spaghetti.