You build stamina for an activity by engaging in that activity or one that's comparably tiring.
I recently spent a bunch of time getting into better shape, much of which involved building stamina. It's pretty simple, but it involves a lot of time. That means you need to want to do it; if you don't want it, the whole thing is going to fall flat.
The thing that worked for me is taking long walks. Some were hikes, and some were just flat terrain. I figured out a distance and a pace that I could do, and I did that a few times a week. The first couple of weeks I didn't push myself much at all. I just did a pace I found comfortable for a distance that didn't exhaust me. It did make me feel tired after, but I wasn't gasping and falling over.
After a couple of weeks, I found that doing the same types of things started getting easier. The hills didn't feel as high and the straightaways didn't feel as long. So I added a mile or so to my loops. If the terrain was really flat and level the whole way, I added two miles. Then I spent a couple more weeks doing this a few times a week.
Key to all of this was finding my own pace and not pushing too hard. I went for feeling a bit tired and slightly sore, not for being exhausted and not for actual pain. I varied terrain types so that some bits were flat and some had lots of hills. And I did it at least two or three times a week, if not four and sometimes five when I felt really motivated.
After a few months of that, 8-10 miles is an easy walk, and 15 miles just needs a bit of stretching after.
This is what worked for me. It might work for your niece. If she's 16, she'll probably have a much easier time of it than me, because I'm more than twice her age. But she still needs to want to do it, otherwise it'll just be a chore you inflict on her.
