First of all, your "main question" contains a lot of small other questions, which I think are difficult to answer as we are not your sister's neurologist (we haven't seen the MRI, we haven't examined her,etc...) But I will try to address some of your concerns:
What are arachnoid cysts?
Arachnoid cysts (AC) are a collection of cerebrospinal fluid (CSF) within the arachnoid membrane. The CSF is produced by the arachnoid cells which line the cyst. The exact mechanism by which those cyst appear and grow is still not clear. The current main theories are passive diffusion of CSF into the cyst or progressive entrapment due to a ball valve effect. The image below (from http://weillcornellbrainandspine.org/sites/default/files/arachnoid-cyst-pediatric-lg.jpg) shows you an arachnoid cyst (the structure in red).
How frequent are those arachnoid cysts?
Arachnoid cysts represent 1% of intracranial masses. In a retrospective study conducted among adult brain MRI, the prevalence of arachnoid cyst was 1.4 percent; of these, only 5 percent were symptomatic. Symptomatic arachnoid cysts are more frequent in children (75 percent of symptomatic arachnoid cysts occur in children)
What are the symptoms associated with arachnoid cysts?
Symptoms depend on the location of those cyst as well as their size.
Most of those cysts (approximatively 50%) arise in the Sylvian fissure (so approximatively in the same region as your sister’s AC), although they may occur in any part of the nervous system where there is arachnoid. Other common sites include: cerebral convexity, interhemispheric fissure, suprasellar cistern, quadrigeminal cistern, cerebellopontine angle, midline of the posterior fossa, and the spine.
Cysts in the Sylvian fissure are usually asymptomatic but can present with headache, seizure and less commonly, focal neurological deficits. Subdural hematomas may occur following relatively minor head trauma.
What is the treatment for arachnoid cysts?
Treatment is generally initiated if patients have symptoms. If mainly asymptomatic, current recommendations recommend serial imaging and neurologic examinations (as suggested by your neurologist).
In general, surgery is indicated if patients suffer from symptoms of increased intracranial pressure, seizures, focal neurologic deficits (= motor or sensory problems in face, limbs) or cognitive impairment. Needle aspiration usually is of temporary benefit and is not a good long-term treatment option .
Your sister has also a retention cyst in the maxillary sinus.
According to Wang et al 1, who followed 40 patients with retention cysts,
Most retention cysts of the maxillary sinus spontaneously regressed or
showed no significant change in size over the long term. These
findings suggest that, in the absence of associated complications,
“wait and see” may be the appropriate management strategy for these
http://www.uptodate.com/contents/uncommon-brain-tumors (unfortunately not accessible if you are not a medical professional or don't have a subscription, but the text in this post is inspired by their paragraph on AC)
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