Well, I dunno, it is her little sister.
Also, if so, that probably means the poor sod she does send will be in quite a lot of trouble (more so than she would be if she went herself). I mean, if Sakura sees Rin there having gone to avenge her, she'll ensure she's freed and given good treatment. If it's someone sent by Rin, then she might not realise what was going on.