Already had to send dd 4.5yo to her room twice today and that was before taking her to nursery!
First time was when I had to tell her 3x not to 'tickle' (rather roughly) ds's face (19m). I do increase the level of my 'don't do xyz' ... 1st time normal, 2nd time a bit firmer, 3rd time very firm with low undertone... at which point she had a tantrum. So off she went into her room...
Then after calming down she came out again and poked her tongue out at me because I 'dared' suggest that ds can watch his TV (Bear in the Blue House' for the next 15 minutes instead of the cr*p she wanted to watch)... so into her room she went again throwing herself onto ds's bed letting rip. Granted she can't throw herself onto her bed as her's the top bunk 
Not to mention all the 'whatever's' she's throwing my way atm.
How long will this phase last?