Hello , I can understand the problem. But you didn t mention the symptoms of your sister.anyway, from your history I can conclude that your sister may be having small kidney stones which is not detectable by routine ultra sound.if it is 3to6mm size very difficult to diagnose by ultrasound. Blood in the urine may be because of stone in kidney. Other causes also there like urinary tract infection, glomerular disease and bleeding disorders. But with your history, spot in kidney by ultra sound and blood in urine mostly favor for small kidney stone. This amount of small stone washed out in the urine itself if you take plenty of water. If you want to confirm the diagnosis you can go for CT scan of abdomen KUB. It can find the diagnosis mostly. Now don t get panic, just ask her to take plenty of water and follow up your gp.