As BarryCarlyon stated, even though twitch has an ‘IRC Interface’ its not really an IRC daemon. It is meant to give the most basic of functionality so bots can have an interface that doesn’t require a whole lot of browser-mimicking.
Due to twitch REQUIRING an account-specific OAuth token to connect you can be assured that the user JoeBlow is JoeBlow and not someone else that has changed to JoeBlow’s nick while he was offline(unlike in a true IRC setting).
As far as DMs/Whispers, there’s already an interface for such, though its a bit obscure. Look through this forum and you’ll find threads related to how to do such.
If you are using mIRC as your platform I have a few scripts that attempt to normalize Twitch’s irc-esq interface on github, including DM/Whisper support.