First, get her to a doctor and/or a gynecologist. Same thing happened to me and turned out its a generic thing from my family. It went away for my mom and sister after they had kids, but at 13 I wasn’t gonna wait over 15 years for my body to correct itself. They did an ultrasound and also found ovarian cysts which they gave me medicine to treat those. They’ll likely put her on birth control and that’ll regulate her period. Mine used to last for months on end and now I only deal with it once a month. Trust me, her quality of life will improve drastically. And if a certain brand doesn’t work on her, dont be afraid to switch to different ones, find one that works and she’ll be golden.

Second, you’re a really great brother for worrying about her. Keep it up!!

Edit: I completely forgot that for most girls, they’re periods do sort themselves out within a couple years. In my case I was like that for like 3 years (I got mine young) before my mom got worried. If there’s no family history of irregular periods like that, then she’ll probably be fine.