Usually, you find that teens tend to move from one relationship to another fairly quickly as they're still discovering and figuring out themselves and what they're looking for. What's worrying is that she's accepting this bad treatment, and it's the guys who are ending the relationship. Sounds like she could do with some guidance. Self esteem sounds an issue. Peer pressure might mean she's more accepting than she should be, maybe as she views being in a relationship more valuable that the quality of the guy she's with. This is bad news for her future, more serious relationships, so she really needs to be guided here(by you, if her parents, older family members aren't able or willing to help guide her. I was married by 19, and my mother married at 17, and so many of us don't confide in our parents etc when relationships aren't what they should be, in order to be healthy for us. Your niece is very fortunate to have you, someone who clearly cares.
As her Aunt, you can gently guide her toward a healthier mindset... the following is from an online Teen Relationship article found here ... https://www.enkirelations.com/teenage-relationship-problems.html
Respect is a key component of any romantic relationship. Both parties should value each other’s opinions and boundaries. Teenage relationship problems come up when one disrespects the other in a relationship. It is important to speak out whenever you feel uncomfortable or disrespected. If he or she is not ready to listen, it is better to end the relationship and move on in pursuit of someone that respects you.