Yeah - this always seemed something of a bizarre account to me.
Even at first glance it looks pretty unlikely, especially if you have had any experience in trying to conceive a baby in a relationship , which often takes several months.
I remember 20 years or so ago looking at the statistics after one of my ( worldly ) workmates raised it in a humorous way.
Given that the odds of becoming pregnant from one act of unprotected sex is about 1 in 20 , the odds of both women becoming pregnant would normally be about 1 in 400. But that is having sex with younger guys. Lot was old ( exactly how old is difficult to determine - maybe 70 or 80?. ) Men just over 40 are 30% less fertile than men under 40. There hasn't been much research on even older guys but suffice to assume that the odds are even more astronomical.
Not to mention the fact that Lot was completely drunk yet able to "perform" on 2 nights running. Or that infant mortality in those days was astronomically high ( living in a cave is hardly the best environment for an expectant mother. ) And having a child by one's father would hugely increase the risk of birth defects or spontaneous miscarriage.
So the "cunning plan" of Lot's daughters might have seemed reasonable at first glance but in reality the odds of their idea working are completely minuscule - 1 in thousands. They would needed to have been impregnated by their father regularly over several months to have any chance of becoming pregnant. Even then it would probably have taken several pregnancies for one to result in a healthy live birth.
The only assumption is that this curious "arrangement" had God's backing. In fact , one elder I mentioned this to felt that this must have been the case. But there certainly is no reason for this to be correct. The Moabites & Ammonites ( from whom the offspring were ancestors ) were enemies of the Israelites & constantly at war with them. So how exactly was an arrangement that involved incest & produced enemies of God's people worthy of miraculous divine backing?
Like so many bible accounts , especially in the OT , this one just doesn't add up. Not that it did Lot any harm. He is still regarded in the NT as "righteous Lot".