Kathy,
I'm sorry to hear how many times your family has had to deal with one form of cancer or another. I don't have an answer to your question about genetics and tongue cancer; however, I can identify a bit with what your daughter-in-law is going through. I was 39, a non-smoker and social drinker when I was diagnosed, and I had had the tumor for a long time (was told repeatedly by my dentist and GP that it was nothing to worry about). My cancer team was determined to be very aggressive in attacking it, and I'm glad they were, as I've had nearly 17 years since then without having to deal with it again.
You don't mention whether she is being treated at a major cancer center (I hope she is, as I think it's advantageous to have the benefit of all the resources that come with those facilities). It's a good sign that her doctor plans to treat this aggressively, as this is not a disease that responds well to piecemeal measures. Many of us here have had radiation, and it can be extremely difficult to handle, especially in the latter weeks of treatment and well beyond that, but she can find plenty of support and advice here to help her get through the tough times.
Please keep us posted as you find out more about her diagnosis and treatment plan, and encourage her to check out this site as well. There's a tremendous amount of very helpful information that can be accessed through the home page, in addition to this forum.
Cathy