Sounds like your game may be a bit messed up. Aunts and Uncles do recognize their nieces and nephews. First cousins do recognize each other.
And your example makes no sense. If Quentin is Elizabeth's brother and Roger is also Elizabeth's brother, then Quentin and Roger are brothers to each other.
I'm linking to you one of my Sims 2 family trees. Give me an example based on the Sims in the tree linked as I'm not following you or your example and have not had any Aunts and Uncles not recognize their nieces and nephews and no first cousins who don't recognize each other.
http://familyecho.com/?p=IQX2B&c=2ehaqcxqfw&f=542884288635395166
Edit to add: Rereading it, it makes me think this is due to siblings made in CAS issue. Is this the case?