Why my sis's blood pressure remains high during haemodialysis and she vomits a lot weighs only 68.34 lbs age 23 renal function end stage?

Well, you will find. A better answer but the kidneys help control blood pressure; if she's losing function it's not unusual to lose BP control. The nausea may be due to several factors but make sure she is on a good medicine to protect her stomach from ulcers. A good, unique drug to help sleep mood and appetite is Mirtazapine at 15-30 mg at night. Depression can get overlooked and she is at high risk. Good luck.
Kidney disease. Many kidney diseases cause high blood pressure since the kidney is one of the most important regulating factors for bp. At 68 lbs, she sounds terribly malnourished or very ill. Vomiting is common with hd and has many causes. Work with your nephrologist to make her more comfortable.