If you could sum up your DIL in two words would they be “gin-soddened”?
But, for your kindliness and the restraint that you have shown over the years, the family relationship would have gone to pot.
You worry that on your death if you leave your money to your son, she will blow it. What if by the time of your death they have split up and her lawyer claims the inheritance as part of the divorce settlement?
You could leave it to your grandchildren but you know what that lot, are like.
One way is to leave your money in your will in trust to your son and grandchildren only. This is called a “testamentary trust”. You can appoint your son as trustee but it may be better to appoint your brother or your lawyer to be the trustee with discretion as to how much to give each year, if anything. If you are feeling cruel, appoint your accountant.
There is a lot of satisfaction in immediately telling your DIL your succession arrangements. But she may seek revenge by trying to commit you to a home at the first signs of dementia.
Therefore, it is best to leave it as a surprise. Alternatively, tell her you are just thinking about it and see if she picks up her act.
But, if this all seems too hard there is a lot to be said for telling her that you are leaving your money to your son in the usual way and then proceeding to spend it frivolously.