Writer's Fallout Shelter

I'd actually suggest starting with literary magazines. So many people self-publish, especially with poetry, and then later they regret it because the poems or writing styles will have changed. If they're finished as far as they'll be now, she can submit to lit mags (most do electronic submissions now). Most lit mags (unless hosting a contest) are free to submit to. The editors look at the content, not the author's background. And your niece might get some great feedback. For example, I started sending out in high school and received personalized rejections about how to fix things and what specifically the editors liked or didn't like. This is incredibly special because it means I was good enough for the editors to take the time to write to me. It wasn't until I got into undergrad courses and grad school that I learned that personalized rejections were not a regular thing. She may encounter the same experience and, despite being rejections, it's an incredible morale booster. As we all know, it's a writing rite of passage to receive a pile of rejections.

Lit mags are also good for finding an agent. Agents skim through many lit mags to find new/upcoming writers. Also, many online lit mags are cropping up either individually or in tandem with print editions, and they may be more likely to accept her work if the magazines are fledgling. Maybe. Some promote excellent literature and may have a harsher eye.

Duotrope.com is an excellent source for finding lit mags. I use it all the time. You can search out magazines and keep track of submissions. Your niece should also set up a Submishmash/Submittable account because more and more lit mags are using that site for submitting.

Good luck to her!