My wife is Thai and we have been married for 6 years.

My visa is Non Imm - O based on our marriage.

What happens to my visa status if my wife dies?

It is possibility, as she is in poor health.

As we are both in our 50's we do not have any children of our own, but there is still the youngest of my wife's children who will need support for another 8-10 years.

If you want to stay after her death you would probably have to change to a retirement visa. If you formally adopt the child you might be able to obtain a visa based on support of the child after her death. It would be best to ask immigration for guidance now so that you are both comfortable and can take any required action (and they have understanding of your situation).

Sounds like you meet the requirement for residency permit. I know it is a little expensive but this might do the trick. You may also look into adoption of your wife's kid?