People who work with kids

I have a 4 (she turns 5 next month) year old niece who doesn’t really have her parents in her life. For the moment, my mom is raising her and they live 3,000 miles away because my mom is also taking care of my grandpa, and I currently live with my dad while going to college. My nieces parents are both unfit to take care of her. Her father doesn’t have a job and my sister is currently doing military training, but she is still not responsible enough to take care of her (she parties and abandons my niece). My niece has really bad tantrums and she throws stuff and hits people. It seems like it’s getting more and more worse. I worry that it’s related to her parents not being with her. Is there any way we could help her to not have tantrums? My niece loves my mom and she seems to look at her as more of a mom than a grandmother.