My husband is in declining health, if he passes away before me , can I have our will changed?

My husband has terminal cancer. We were married five years ago, second marriage for both of us. When we had our will made, we divided everything equally between his two " adopted" adult daughters and my two biological children. In recent months his adult daughters have proven without a doubt that all they are after is money. His, mine and whatever they can get their hands on. I recently found out that they were foster children that he took in as teenagers and were never legally adopted. He refuses to change the will. My mother is very concerned about my future inheritance from my family, getting into the hands of these two woman. Any suggestions? I actually considered divorce.