Seems plausible to me, assuming that my situation on 23andMe is about average when it comes to the number of DNA relatives and the vulnerability of my relatives

Seems plausible to me, assuming that my situation on 23andMe is about average when it comes to the number of DNA relatives and the vulnerability of my relatives to being hacked. A quick search says 23andMe has 14 million customers, so 14000 accounts breached would be 1 in 1000 accounts breached. The DNA relatives listing for me lists just over 1500 people. If each of those accounts had a 1/1000 probability of being hacked, the probability none of my relatives were hacked would be (1-1/1000)^1500 = 0.223. The probability that at least one of my relatives was hacked would then be 0.777. I'd then expect, based on my assumption that I'm typical, about 10.8 million people to have had relatives with hacked accounts, which is close enough to 6.9 million that the latter seems plausible.