According to the official website, four extra episodes of Ore no Imouto ga Konnani Kawaii Wake ga Nai will be released in the 7th and 8th volumes of the Blu-ray & DVD.

Sources: OreImo official website, Scan of Animage

Confirmed!!
At the live event held on December 5th, Aniplex has announced that the TV series of OreImo will be concluded in episode 12 as "Good End", but a "True End" arc will be branched off at episode 11 and will be streamed online. Producer Iwakami Atsuhiro said they will do their best not to postpone the streamings like Bakemonogatari.

Flowchart of the branches
TV series: episode 11 -> episode 12 (final of the good end arc)
Online: (episode 11) -> online episode 12' -> episode 13 -> episode 14 -> episode 15 (final of the true end arc)

Source: OreImo official website