http://www.ldsces.org/inst_manuals/ot-in-1/ot-in1-03-gen-b.htm
Genesis 19:30–38. The Sin of Lot’s Daughters
The account of the incestuous seduction of Lot by his two daughters is a shocking one but one which, again, illustrates that the Old Testament records the evils of the people as well as their righteousness. There is no way to justify the wickedness of what the two daughters did, although it may be better understood when it is considered that the daughters may have thought that the whole world had been destroyed in the holocaust that befell Sodom and Gomorrah and that Lot was the only source of children left to them. Moses may have included this account in the record because it shows the beginnings of the Moabites and the Ammonites, two peoples that would play an important role in the history of the people of Israel.