My boys are 3 1/2 months apart - thankfully in different school years so only once have had someone tell them they must be mistaken about not being twins. Few people listen to the answer after asking about the age gap, just nod and smile - though most are fine with "I only have birth to one of them" with the exception of a colleague who retorted, "Well I could tell that! I am a qualified vet, you know!"

Actually there's a whole angle for fic - Peter is actually adopted... Is he really Aunt Molly's child?

I think I'll keep Peter one year behind Ginty and one ahead of the twins (even if they should be in his year but repeated one), so making his birthday in late September would work.
Close enough for government work, as they say...