I'm pretty sure that was an indirect way of saying "that's really irritating, please stop." It's really irritating, please don't capitalize every word.
Anyhow. I think I would be a little freaked at first but then when I realized what happened I'd probably laugh. I take it your sister is fairly young? I would be all "D'AWWW, that's so cute and childlike! n___n!" But I love my sisters to pieces. They're so cute and naughty! I'd explain it to her and probably end up tickling her until mom got mad x)!