Dear Francesca:
Sorry to hear about your sister. I would wonder about the etiology of these two subdural events. Is there an underlying cause? or just extremely bad luck. I would thing one time, okay bad luck, but twice I think that there may be something like AVM or the like. If this were my sister I think I would push it alittle (one thinks of von Hippel Landau).
I would initial try to see it the archnoid cyst goes away on it's own. But I would not wait too long, then if nothing happens via MRI I would initial try and drain it. If one drains it and it does not return then your in luck. As you know shunts presents more problems, but it it is needed then so be it but I would use it as a last resort. Maybe a drain then injection of interferon.
Sincerely,
CCF Neuro MD