Is it within code to branch an exterior outlet off an interior outlet in the middle of a circuit?
The outdoor receptacle will have to be GFCI protected.


  NEC 210.8 At dwellings, ground-fault circuit-interrupter (GFCI)
  protection  shall be provided for all receptacle outlets installed in
  bathrooms, garages, grade-level  portions of unfinished accessory
  buildings, crawl spaces, unfinished basements, kitchen  countertops,
  wet-bar sinks, boathouses and outdoors. Receptacles that are not
  readily  accessible may be exempt from the GFCI requirement


The receptacle will also have to be contained in a weatherproof enclosure, and if something will be plugged in for extended periods (Christmas lights for example) the enclosure should be labeled as "watertight while in use".


  NEC 406.8 15 and 20 amp, 125 and 250 volt receptacles installed
  outdoors in a wet location shall have an enclosure that is
  weatherproof whether or not the attachment  plug is inserted


It should not be a problem branching from an interior circuit, but you'll want to seal the hole where the cable enters the house to prevent moisture from entering.