Though it's wise to consider whether you want to have a sequel before you overdo the fertility.
If you give the fairy tale prince and princess half a dozen children, you might not have to write half a dozen sequels -- if you're following the fairy tales, you won't, where one child in the family has a story -- but it would be an odd novel indeed to not at least allude to the siblings, and to do something more with them than making them all the opposite sex from the main character, or older so the main character can be the youngest.
poke, poke, poke
I think marrying off a few of the siblings first would help. Though not perhaps in the Black Bull of Norroway style, where the oldest goes off in a coach and six, the next in a coach and four, and the youngest -- our heroine -- with aforementioned bull.