January w/ 5:
DD,
a niece on my side,
a niece on DH's side,
DH's mom, &
then DH
We had a sister-in-law who passed away whose birthday was also in January, so it used to be 6. (And both my sister's bday & DH's brother's bday are at the tail end of December.)
Our next closest is July w/ 3 bdays - mine & our 2 DSs' birthdays.
What's neat is that in our immediate family, DH's & DD's birthdays are both in January while mine & our 2 sons birthdays are in July, & it was totally unplanned. The birthstone for January is a garnet which a deep red, & the birthstone for July is a ruby which is a lighter red - so, if I ever get a mother's ring, it'll be a mix of garnets & rubies.