My sister wants to go back to a man who was horrible to her and refuses to see things from my perspective. I know she will only get 
hurt and I will have to console her. They were together for two years, broke off their engagement, and she took a long time to get over it, which is why her decision is so devastating for me. It’s as if she learned nothing from the episode and is only thinking about herself, instead of the pain we all had to go through on her behalf. I am tempted to stop speaking to her until she comes to her senses, but I also know that won’t help and may only push her closer to him. How do I get her to make the right decision?
What may seem like the right decision to you needn’t be one that works for her because your sister is an individual with a mind and identity of her own. Your reasons may be valid, and your love isn’t in question, but sometimes love means being able to support people despite and precisely because you know they are making a mistake. These are two adults trying to make things work, and it may or may not, but your role as a sibling is to be there no matter what. If your sister knows she can count on you, it will only give her the strength to make the right decision when she is confronted with one. Tell her you are on her side, and ready to talk about anything if she wants to. At this point, that is the best thing you can do for someone you care about. Everything else is out of your hands irrespective of how badly you may want to control this.

Am I naïve for believing in romance novels and hoping to find a partner based on them?
Love stories are different for different people, and there’s nothing wrong with hope or optimism because we can’t see into the future.