Septimus Heap/Headscratchers

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.