why does this keep happening?

I'm honestly at a loss for my poor sister right now. She cannot seem to catch a break. She is 29 and since she was 17 has been in 3 serious relationships. The first was with a guy named nick. He had steady work with the family business that he was in line to inherit when his dad stepped down. Seemed to have his life in order. He got hooked on pain killers after surgery and became abusive when he wasn't high. My sister finally left him and fell in love with Juan. We loved Juan. He had a son around my brothers age and they got along and he just treated everyone in the family like gold. Come to find out he was cheating on her and was abusive. She got out and moved from Chicago to Milwaukee with me. Got a fresh start. Met a guy named mike and although they bickered and such like normal couples they seemed happy. Families were integrated. My sister moved into his house with him after 2 years and he proposed. They were planning the wedding. We literally just had family Christmas at their house on Monday. Well I got a call last night and my sister is sobbing. He told her he isn't happy and she's just fat. He has been talking to another girl and they've had sex and he'd rather be with her. End of story. So my sister is tearing apart the house with my mom right now packing all her shit. My dad is going there tomorrow to help load any furniture. I just don't know why my sister keeps going through this. This time is came out of the blue. Last week they were planning their wedding and talking about when they should start trying for a baby. My heart just breaks for her. She's such a wonderful person and deserves so much more. I'm just not sure what to say to her. I don't know how to comfort her. Any kind words, thoughts, or prayers for her would be much appreciated right now.