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 Sally was shown doubting Silas was the father, but never questioned that Sarah was the mother.
Marcia also considers all babies/children to look alike.
Apart from the difference in appearance, which Marcia does kick herself for, the Heaps were the perfect adoptive family for Jenna. They're an old magykal family with powerful allies like Aunt Zelda and the Wendron Witches who they could call on for help, as well as strong personal ties to the Wizard Tower, so Marcia can keep an eye on the Princess and intervene if she has to- which she does. Despite this, they live a very simple lifestyle in the Ramblings, which are basically crammed full of weird and interesting people, essentially making Jenna a needle in a haystack. The baby princess also needed someone capable of nursing and caring for a newborn, which the Heaps were well able to do as Septimus literally shares a birthday with Jenna, and which as stated above gave them an easy way to explain suddenly having another child. Finally, putting Jenna with the Heaps ensured that she would grow up happy and loved, and instilled with values that would make her an excellent Queen. Really, the best thing Marcia ever did to ensure Jenna's safety and success was to leave her with the Heaps.
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?
In-book, Jenna didn't want to risk bringing up too many people, and the Extra Ordinary Wizard at the height of her power is a better bet than a fourth-year apprentice, Magical Seventh Son or not. Out-of-book, it annoys me too.