high blood pressure
can affect the health of the mother and also in sometimes it can lead to intrauterine growth retardation
of the baby as your sister is having,
so as continuation of pregnancy
can lead to more health problems like eclampsia to her and can further affect the baby, in case of uncontrolled blood pressure better to terminate the pregnancy, usually by 32 weeks with 2kgs weight baby can survive well, if needed you can take the help of neonatologist.