If you are supposed to have the girls 50% of the time and it is your ex husband who is not allowing you to do that, you could file a motion to enforce the parenting plan. If he is willfully violating the parenting plan, you could also file a motion for contempt. Those both can take some time, though, so he may not be alive when the hearings happen. If you currently have joint custody, and their father dies, you would have sole legal custody (barring some other court action out there that might have impacted your legal rights). If you do not know where your daughters are, though, you will need to find out. You should consult with an attorney privately. It sounds like their is more to this story - so you should get specific advice based on your case.
Shannon L. Hall, Attorney at Law (Licensed in Oregon); 960 Broadway Street N.E., Suite 4, Salem, OR 97301; Phone: 971-209-2443; 245 East 4th Avenue, Eugene, OR 97401; Phone: 541-434-2411 My response(s) to the question(s) on this website do not create an attorney-client relationship. An attorney-client relationship is not created until a Fee Agreement has been signed. In addition, my suggestions are based on very limited information provided by the Asker and are given based on my experiences and general circumstances. My suggestions may not ultimately be applicable to the Asker's situation because of the limited amount of information provided. No suggestion is guaranteed to be sucessful. Case specifics should not be shared online. You should seek specific legal advice in a private setting.
I would add the following. You stated in your question: "He is terminally ill with stage 4 brain cancer and was given 3 months to live." He has taken the children, you do not know where they are, and he has failed to notify you or the court regarding his change of address. Obviously, I know nothing more than what you have told us, but you might wish to advise the police or DHS about this and get some input from them.
Sign up to receive a 3-part series of useful information and advice about child custody law.