So, there are moral questions that the bible leaves untouched. Interesting. How do we determine if some action is moral if it's not covered by the bible?
There are principles taught in the Bible that provide guidance. The sanctity of human life for example, together with scripture that seems to indicate life begins at conception provides guidance for both abortion and stem cell research. There is the principle of love your neighbor as yourself.
You use your head. You pray. You take counsel together as a society. And if the community is not too spiritually deprived, they ought to be able to come up with a moral solution.
Another good example is your maternal uncle marrying his niece. Since scripture repeatedly emphasizes that uncles and aunts are "near kin" as the reason for not marrying, then a prohibition on maternal uncles and nieces marrying although not directly specified should be a no-brainer.
Modern genetic studies would weigh in on that as well showing that the risk of genetic defect is higher. Under the principle of love your neighbor as yourself, you don't want to risk a 5% chance of bringing a baby into the world with a genetic defect.