Why do family birthdays seem to cluster together? Are there genetic cycles of fertility? My niece, Helen, turned ten on Wednesday; today is my sister's birthday; mine is in three days. My in-laws have birthdays mainly in late autumn except for my husband and our nephew who share a birthday in early May. It's odd.
It works out nicely, though. Aside from being convenient for gifts and cards (I never forget these birthdays!), it can be fun, too. When I was 9 and Jenn was 6 we shared a birthday party. My friends sat on the side of the cake that showed 9; hers were seated so they could see 6. I don't think we can do that number trick again, but maybe someday we'll share another party.
I think it's just a coincidence in your family. I know the birthdays in my family are not like that, along with many others I know. I highly doubt it has to do with anything scientifically. After all it's up to the couple when(around when) to have their child.