She takes tension about things or gets stressed and anxious, which is one of the things disrupting the menstrual cycle. Speaking openly to friends/family will be good.

Remedies (have to be done by your sister):

1. Fast for 11 Wednesday’s, start from Shukla paksh Wednesday (check rules on internet. One thing is don’t eat salt in any way).

2. Plant 2 or more tulsi plants at home and take care of them. Chant Gayatri mantra or Sri Vishnu’s names while giving water to tulsi maa. Simple rules related to tulsi plant are there, do follow them.

3. Do this everyday if possible, but if she cannot do it everyday then she has to do it on every Monday and every Chaturdashi at least – abhishek on shivling with milk and then water (separately). Can be done at home or temple. Chant “Om namaha shivay” while doing abhishek. After it is done, sprinkle few drops of the abhishek water+milk on self.