It's not that simple. If the allegations were very serious, like sexual assault or something, then sueing immediately would have been an appropriate answer. But in this case, the allegations sound so mild at first that you might think it's not worth the trouble. Of course knetz are mad nonetheless, but really we're mostly talking about an argument on the phone. Are you gonna sue over this?
For all we know, even the older sister might be misled by her younger sister. Who knows if back then she didn't blow things out of proportion and now has to stick with it just so she doesn't end up labeled as a liar?