When you love someone, you accept them for who they are and everything that comes with them, whether it be their children or their history. There have also been instances when step-children became so close to their step-parents that their relationship was indistinguishable from their real parents.
Even the original poster (OP) loves his step-daughter, so it hurt him when his fiancee ignored his clothing advice for a kayaking trip, and said that she’s her child, not his. When things went south, she demanded that he fix it, but he gave her a taste of her own medicine!
More info: Reddit