My sister is older than me, so she'd probably go 'Oh... Sorry' and just leave. Or, come in and go 'Ah, sorry. Get some.' and then walk out laughing awkwardly.
Unless the probable situation was happening, being we're both playing halo or something is happening, in which case she'd probably just say what she wants, and then leave
I'd probebly tell her to get lost. Not that i don't love my sister....but i do hate when people walk into my room without knocking and asking nicely.
And then she'd probebly leave without realizing something happend.... or if she did, she wouldnt comfront me about so meh. /: