"Answers to your Questions!--No.8" (DFO 1566), September 1983, on page 115 of "Good News" Book 11 says:
8. QUESTION: (MARIA: IS THERE ANYTHING AGAINST INCEST IN THE BIBLE?)
ANSWER: SURE, THE OLD MOSAIC LAW, WHICH IS NO LONGER IN EFFECT! (Maria: I thought there wasn't anything in the Bible against it?) It's not really clear in the Bible, but there are plenty of man's laws against it, even the marriage of first cousins! Incest has been known to bring out either the strengths or the weaknesses of a family, & it often produces either geniuses or idiots, just like any other kind of inbreeding. But with God in control, there's no problem but man's anti-God laws! Cain & Seth obviously married their sisters, & Lot married his two daughters without any problems!--But watch out for man!
In the same Letter, 28 pages later, appears the shocking question by a woman disciple, followed by Berg's even more shocking answer:
100. MY SON & I HAVE A VERY CLOSE & LOVING RELATIONSHIP. Is there a risk of producing abnormal offspring if son & mother make love? Was there a specific reason that it was forbidden under the Mosaic Law? ANSWER: SEE QUESTION NO.8 ON INCEST.