My Sister Sugars Is Over 500 And Doctor Can't Seem

My sister sugars is over 500 and doctor can't seem to get it down. and they believe it could be an infection in the body. Having sugar that high for a long time,can it cause infections in the body? she has been losing weight ,teething some of her sight. She don't have insurance and doctors they providing is not getting meds needed but trying to figure out why sugar want go down and now she develop high blood pressure on top of it.
What can she do while she wait for an answer from doctor? So she want have a stroke.