My brother recently adopted a seventeen year old girl, and she’s one of my favorite nieces, but I’ve reached a dilemma. I’m marrying her older sister, and now I don’t know what to call her. She’s technically my niece, and my sister-in-law. What do I do??
You can call her both, because she is both. Though alternating between the two could be a bit abnormal, but then again, this is a site for grammar and not discussion of social norms. But you'll have to consider the context for the situation where you're referring to her. In some contexts, her being your niece is the most relevant factor, while in other, her being your sister-in-law is.