I have done a DNA test and show a 2nd cousin match with 313 cM and then a 4th cousin match with 67cM yet these two matches are sisters with the same parents.
How is this possible?
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It's possible because the relationships the DNA company gives are estimates and because every actual relationship has a range of cMs that can match it. Relationship estimates also use things like segment size. A couple large segments (over 20 cM) indicate a closer relationship than several small segments (say, 5-9 cM), even if the total cM is the same.
If you were a 2nd cousin to these sisters, you could have a cM range from 46-515 cM. This fits well with your 313 and 67 cM matches. You are not 4th cousins.
The fact that you have two matches who are full siblings is great because it really narrows down the possibilities. 67 cM is a meh match. But 313 cM is terrific. Just the luck of the draw (the shuffle of the genetic cards) how you match someone. And how interesting that you got both ends of the range.
Based solely on the cM matches with both sisters, here are some possible relationships between you and the sisters, with the ranges given:
(Note: some of these relationships are unusual enough that there just wasn't a lot of data, which is why you get weirdnesses like a larger range for Half 2C2R than for Half 2C1R.)
You can narrow down this list with paper trail genealogy, testing your siblings or other siblings of the sisters, and testing other close relatives of yours or theirs. Now you know why people into genetic genealogy push people to test your siblings!