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.
I don't know when we got our SSNs. I think they started being issued for tax reasons around that time, but don't know for sure.
The second two numbers I think are based on zip code.