Honestly, the numbers are a total headache when you're starting out. Basically, the X-Pro3 has a sensor that makes everything look a bit more zoomed in than a traditional film camera. For portraits with that nice blurry background, you generally want something between 35mm and 56mm. The lower that f number on the lens, the more blur you get. If you want the best bang for your buck under 600 bucks, you should definitely look at the Fujifilm Fujinon XF 50mm f/2 R WR. I've used it for years and it is super sharp, plus it's small enough to not look intimidating at a party. It handles portraits beautifully because it compresses the background just enough. If you really want maximum blur though, the Sigma 56mm f/1.4 DC DN Contemporary for Fujifilm X is probably the better bet for your budget. Since it hits f/1.4, it lets in way more light than the f/2 lens, which helps if the party is indoors or in low light. The 56mm focal length is perfect for headshots or upper body shots. Just keep in mind that with a prime lens, you have to move your feet to zoom in or out. It takes a second to get used to but the image quality is way better than a basic zoom. Dont overthink it too much, both of these will make your niece look great... it mostly comes down to how much room you have to step back at the venue.