Thanks to the list I now know I can add a disconnected individual that is a sibling when the parents are unknown. However, I don't know how! I had to create a way to do this. Can someone please tell me if it's ok they way I did this?? Seems to be.
I typed in the sibling name and made it active in the RELATIONSHIP view. Next to PARENTS, I can click "add" and a message states that I can "add person as a child to existing family". However, it does not let me actually DO this. GRAMPS v3.0 wants me to then select a family from a pre-determined list of names instead of "Blank" and "blank" (parents unknown) which is the family on my screen.
So I created a "name" for the unknown dad and made that name "question mark, (?)" Once I created the dad's name, which is just really a question mark, THEN it let me add the disconnected individual as a sibling to the active person.
It seems to work ok this way and unless someone sees an error in doing this this way, I'll continue adding other disconnected siblings to their question mark fathers after I create them. I hope this makes sense!