Thank you, ksam. My sis was naturalized in 2001. The age difference between my sister (petitioner) and brother (beneficiary) is 3. I became naturalized on "EB-1 based greencard plus 5-yr residence after greencard" basis last month.
For the DNA, I already arranged for "Legal DNA" option on easyDNA.net - they'll send one kit to my sister (US) and the other kit to my brother (India); both will go to their respective doctors with IDs, doctor would certify identity, give samples in sealed envelopes and the lab (which is ISO and AABB acredited) will send us report, which we'll send to the USCIS. The RFE did not mention about how/where/what type of genetic tests/lab.
Regarding parent's marriage certificate, since we don't have one any way and none of the parents are alive, affidavit is the only option. They don't give "non-availability certificate" for marriages (they do for births). Hopefully, they will be satisfied with the affidavits.
Our cases are straightforward (or so we thought), but looks like random draw/luck as you said. Thank you.