My niece's birthday is 12/22. My mother-in-law's is 12/27. My father-in-law's is 1/7, and another niece's is 1/15.
I've never seen my 12/22 niece get a combo gift. We always celebrate separately, but not always super-close to her birthday. The family is getting together this weekend to celebrate her birthday this year.
MIL has never liked her birthday, has complained about combo gifts in the past (doesn't get them from us), and complained that her birthday got overlooked when she was a child. It bothered her then, but now she doesn't seem to mind, and often says "let's celebrate later."
FIL and 1/15 niece like having January birthdays. Maybe they'd like summer birthdays more, but they both think their birthdays are far enough from Christmas that they don't get combined... But they think it's neat that they can see what everybody else got for Christmas, then say "you know, I think I'd like one of those for my birthday."