The next two numbers indicate the time period. (My sister & I were born in 1970 & 1972 and the first 5 numbers of our SSNs are identical.)
It always bothers me that people give out the "last 4 numbers" of the SSNs not realizing those are the only real unique identifying numbers in their SSN.
They are, or were the year you got the card. Did you and you sister get your cards the same year?
My wife is 7 months younger than I am, but got her SS card two years before I did. (We're OLD, well before you had to have one for school, or at birth) She was babysitting and needed one to file her taxes. I didn't work until after I got out of high school, and so didn't get one until just before graduation. Her "next two" are two lower than mine. Our first 3 are different as well, but both are correct for the state we lived in.