Sorry, but it is coincidence.<br />
In general, birthdays 5 days apart are not much more or less likely than any other interval - so a 1 in 182 chance. There may be a slight seasonal factor making closer dates more likely (eg increased fertility).<br />
Each zodiac sign has around 30 days, so that by the time you get to 5 days apart there is quite a strong probability that you would be in different signs (I think about a 1 in 6 chance).<br />
So neither is particularly unlikely. Within a group of 1,000 people with a brother, you would expect one to be in exactly the same position (plus some with closer dates who were born under different signs).<br />
Incidentally, I thought it was worth pointing out that there ought to be 13 zodiac signs. The zodiac signs are lined up along a line in the sky called the elliptic, which is the path followed by the sun, the moon and the other planets. This line passes through a 13th constellation called Ophiucus (the Serpent Handler), but mostly we count the time when the sun, moon or planets are "in" this constellation as being in Scorpio.
it would have been fate if the two of you were born the same year...5 days apart. ..
my sis bithday is 6 days away from mine too... but shes 2 yrs older