When my wife and I got married, we both had property. I sold mine and we live in her property. We now have been together for over 11 years. I invested many thousands of dollars in her property as she requested "To make it my home", but I find now that the total property value will go to her son from a previous marriage and the money I invested will go to her previous family. I also have children from a previous marriage. I want to buy some property (land,etc.) so in the event I'm left alone I have a place to go or if I decease that my children would inherit my property. My question is " How can I legally buy property now, so my children will inherit what I have and she cannot come back later to try and claim it?"

Depends deeply on your state, perhaps bought in trust, or bought just in your name and theirs now. You need an attorney to review your current situation.

Also if your wife had you invest a lot of money into your current home, you should have some demand that your name be added, or at least that you be granted in will the right to live there until you die.