Well perhaps that's the problem then, though I didn't think of that. It makes sense that since Siri is the youngest of four, if Vivena is only 20, Siri must be what - 15 or 16 at the most?
In chapter 2, I wouldn't guess 16. I would guess closer to 14 probably. She doesn't seem to have any respect for anything, her father, her siblings, even her religion (which I don't blame her for, crazy Vulcan religion if you ask me). I would feel better if there were one thing pointed out that she respected in her life. Like, just to throw something out, she's 14, so maybe she loves horses, right? Maybe the calmness of being with a well-trained horse helps her center the emotions she can't control the rest of the time. She doesn't even seem to feel particularly bad for "flaunting" her emotions the way she does, or "drawing attention to herself" which is fine, but I feel like she ought to have one stable thing that she does conform to, or the rest is just meaningless. Her "rebellions" aren't even particularly exciting, she just doesn't want to sit in the house staring at grey walls all day. (Again, don't blame her,) but she could be throwing eggs at the stableboys or pouring hot wax on the stairs.
In chapter 3, I don't have any sense of how she's supposed to act because I don't know her at all... and yet I feel her reactions are off. Does she have an unmentioned dead mother to miss? (Or did I miss her dead mother?) What is she going to miss about home? Who is she going to miss? If she's 15, there's NO way she's likely to be ready to get married, (Unless they marry kids off younger, which is admittedly possible in the pastoral society, but none of her elder siblings is married yet), she should be terrified at being forced to grow up more than anything else, given her former behavior. She brings up that Vivena was better educated, but how much was Siri educated? If she's 15, shouldn't she still be in some kind of schooling? (Even remedial religion, something like that, especially for her.)