A typical post-lumbar puncture headache is better when a patient is lying flat, and increases with sitting or standing. Based on your description, this does not appear to be the case. It is possible that this started as a post-LP headache, and is now different. Viral meningitis can cause recurrent severe headaches during the recovery phase. Also medications and changes in diet could have triggered migraine like headaches. Without speaking with your sister, examining her and reviewing imaging I cannot make a clinical diagnosis. Perhaps a second opinion with a headache specialist would be reasonable. Good luck.