Your brother or brother in law
Great Uncle.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
None. There is no relation through "my friend"
Nephews and/or Nieces
There really is no relation, but if the aunt wants to call that half-brother something, call him a nephew. It's only polite.
Uncle if you are male and aunt if you are female.
Your fathers brothers are your uncles. You are their nephew, or niece.
2nd uncle I believe
3rd cousin
You are their great aunt/uncle.
Nephews and nieces
your nephews