This has bothered me since day one: Jenna isn't a Wizard (in a universe where Wizards and Muggles are distinguished by something as conspicuous as eye colour), the Heaps have already a plenty of children to feed, and yet Marcia chooses them as Jenna's foster family. Isn't this risking several disasters at once?
I think she chose them just because she knew them well enough to know they'd take her in and look after her. Maybe she doesn't know that many families and they were the best she could think of? Or, she may have been thinking that since Sarah was already heavily pregnant, they could pass them off as twins, which would look less suspicious than a woman suddenly having a baby out of nowhere. Which did kind of work; Sarah's friend (can't remember the name, been a while since I read the books) was shown doubting Silas was the father, but never questioned that Sarah was the mother.
Marcia also considers all babies/children to look alike.
After Wolf Boy and Jenna get burned in Flyte, Aunt Zelda takes care of them. Thing is, she bandages Jenna's smaller arm burn before attending to Wolf Boy's seared palms. Why is that? Surely hands are more important than arms at least in terms of needing to heal faster so that full movement is retained.
Jenna is the (future) Queen, and as the Keeper, Aunt Zelda is very much her servant and protector. So it makes sense that she'd focus on Jenna first, even though her injuries aren't as bad as Wolf Boy's.
Why would Septimus just stand around and do nothing in the climax of Fyre? Sure, he solved the pyramid thing, but that's not something a main character (and in this case, a titular main character) should be satisfied with. This is the big boss battle of the series that has been built up to since the Two-Faced Ring was introduced. And yet Jenna, not even a queen yet, tells him, the Magical Seventh Son who could at least support Marcia with more magical power if needed, to NOT help?