From the description you have mentioned, it seems that your sister is having Acute Gastroenteritis
which can be due to taking some infected food or drink. You should take her to emergency department and get her examined so that treatment can be started immediately to prevent dehydration
and to cure the infection. Treatment includes antacids, antiemetic, antibiotic and antiamoebic.
Give your sister electrolyte rich drink to prevent dehydration and electrolyte loss due to excessive vomiting
I hope this is helpful to you.