Why can this not be "Il et mon père..." is it when you split up subjects like this you have to use moi, toi, lui?

I think it is because we are talking about a indirect object. qui? lui

I found it. About.com says disjunctive or stressed pronouns are used when a sentence has more than one subject or object:

http://french.about.com/od/grammar/a/pronouns_stressed.htm

I'm wondering why the possible answers are "...et lui sont des freres" or "... et mon pere sont freres." Why des for one but not the other?