I think George RR Martin has made it a career to kill off characters. Yet every single time he does that advances his plot in some way.

If you have a character that only pops up because you expect that girl to be part of a pair, why is she there at all? Each character in a novel (another way fiction differs from real life) is that each character must have a purpose and move the story forward. If she's weighing it down, get rid of her (and that doesn't mean kill her off, it means write her out before you begin).

My first book had a main character with an older sister and younger twins. Seisa-sleepingcatbooks.com (good friend and has since become an editor) told me very helpfully that the twins didn't have personalities. They moved one scene forward, and it wasn't very strong. So it was like they existed in the book for one purpose - to break something later in the book. I didn't need them and they cluttered the story. So in my rewrite, I took out the twins (they had names but I don't recall them anymore). I had their one big moment of breaking the important thing and made it mysterious so it could have been done by the main character's friends or rivals in several different ways, and no one is really certain what happened. Because of that, the book was a lot stronger, the home scenes came together without the burden of the extra characters that weren't properly real and the confusion in the breaking scene came with having so many people to pin the problem on without knowing who did it.

Every character has to count in fiction. There are no coincidences.