If my daughter has ovarian cancer and is divorced, if she remarries how do I exclude her new husband from my Will?
You both can and can't do this.
1) You can easily exclude him in the sense of not naming him in your will; that is, you don't have to give him anything.
2) However, you don't have any control over what your daughter does with her money. So, for example, if you leave money or property to her, since she is married to him, he will obviously get at least some benefit from it, and nothing stops her from spending it all on him or giving it all to him to do with what he wants. Similarly, if she inherits from you, then she passes away, the money, etc. will then presumably pass to him.
3) You may wish to consider a trust as the way to exert some control. You can set up a trust which either pays certain limited expenses for your daughter (e.g. medical costs) and/or doles out the money to her on a controlled basis (some percentage every year) and which is set up so that upon her death, the remaining assets in the trust will be distributed to people other than the new husband. Only by controlling how the money gets to your daughter can you effectively keep it away from him, since once it's actually in her hands, it is her choice what to do with it.
You should consult with a trusts and estates attorney to explore whether a trust would serve your wishes. Good luck.