DISLAIMER: IANASBIRTSMFHS (I am not a scientist but I remember the scientific method from high school)
You don't "know" of that.
You believe your birthday is March you don't know it. You were there at the event of your birth, but unless you checked a calendar and clock and have total recall you dont know it. You take it on faith that no one has messed up with the facts. Proper calendar on the righ page, clock set and giving correct time. Besides, what is correct time? for that matter what is what is "March"? Who's frame of reference?
Are you sure she is your sister? Are you positive? Did you do a DNA scan?
As for getting up early in the morning, how do you know that a huge comet, massive earthquake, a massive layoff at your company, Winning the lottery or some other event won't eliminate your "need" to get up early and go to work.
You need to come up with something else to disprove the grandparent's "theory".