I am so sorry for your loss and for the situation.  Executors are named in a Will so if you are indeed named then all you have to do is file a Petition to be named.  If they object (they have to be served with notice) then that is fir them to do, not you.  If there is no Will then you can file to be appointed as the Personal Reprensentative of the estate (generally called an Administration).  Your half siblings may also have the right to be appointed (if they are also his children) so if they object they can, again after notice, and the court will decide.  But starting the filing is the key.  Get help.  Good luck.