My sister (24 y/o) woke up at 3 a.m. with bad dizziness that worsens every time she turns her head. shes also gained some weight, and her period hasnt shown up for 4 months. my mom has vertigo. what was the cause of this? how can i help my sister?

Probably vestibulitis (inner ear viral infection). If 1st episode, Urgent Care (or Primary Care) can help. If long history of similar would start with ENT (ear doctor)

Amenorrhea in her age group is a little concerning. It may or may not be connected to the dizziness. I assume she's taken a pregnancy test...if not, she probably should. Most dizziness in her age group is benign but there's always the possibility of a "snake in the grass". She probably should have a good physical exam before blowing it off as "vertigo".