As the person who chose that wording:

It's not supposed to be a serious remark. People have been been referring to PIPA as the little sister, merely because SOPA is the more commonly known piece of legislation. (I chose to keep SOPA the focus, even though it's been temporarily shelved, just because it's the signature phrase in this whole ordeal.)

Also, I think it's kind of funny to call PIPA the evil little sister, and I hope a little levity makes this grave ordeal easier for everyone.