Way way too early.
Could be grandpa and grandma killed daddy and his cousin.
Way too much not known. Three of things have to be present 1 ability, 2 opportunity, 3 intent all to do grave personal harm or injury. Not necessarily to dad he can protect his self, family, or depending on jurisdiction his/her property. Two guys not armed against an old man are a grave threat under certain circumstances. Info is too thin in this story. Now we need to know what really happened.