Haemangioma of the vertebral part of the body is often an incidental finding in MRI of spine. Per se it has no significance. Occasionally however, it may be implicated as a cause of pain. The finding on MRI has no significance unless surgery is required to be done on the vertebrae. In that case, there is a possibility of severe bleeding. Otherwise, you don't need to worry at all about the diagnosis.
In all probability the cause of your sisters pain is disc prolapse at L5-S1 level. 80% of pain or disc prolapse often settles with pain killers and restricted activity. There is no serious problem to her health. About 10-20% cases may need surgical intervention if the pain does not get resolved.
Surgery is reasonably safe and the patient is up and about within 24-48 hours.