Re: Why take joy in hurting others?
I find it very difficult to believe that sisters treat eachother like this.
As-salik, have you ever sat down with your sister and told her that this is how you perceive her to be? If not, do so now, make her realise that she is your sister and sisters should never want to make eachother feel bad.
I don't get how sisters can be jealous of eachother, you know there is a hadith "Want for your brother, what you want for yourself" or something similar, same applies to sisters.
IF she is doing this because of jealousy, she is INSECURE and you may need to explain to her that she has no reason to be insecure, reverse the psychology, tell her her good points, re-iterate them and then tell her yours. Say you have your good points, I have mine. Tell you still consider that you need to improve on yourself, and she should also improve on hers. May be she will realise that every single small thing she says to you has an effect that is unpleasant.