Dear Ang,
I feel for you as you are clearly so worried about your sister. It is easy to get the impression that everyone with arachnoiditis has a grim life and is very disabled. This is partly because many of the online groups have a lot of members who are on the severe end of the spectrum.
The bottom line is that none of us have a crystal ball, and that arachnoiditis is incurable. BUT (and this is a big but) many people manage a good quality of life despite the condition and only a minority become severely disabled.
Generally, the pattern is that after an initial period of deterioration (usually linked to some event such as surgery/trauma) there is a 'plateau phase' in which there is stability, albeit with ups and downs ('flare-ups and remissions') much as with many chronic inflammatory conditions.
The people who stay as active as possible and take sensible medication on a regular basis (enough to give some pain relief but usually not complete eradication, at the lowest effective dose) tend to do best. This requires a broad-based plan tackling all the different problems, so one mode of treatment (e.g. tablets) is generally not enough to make a big difference: it takes a combination of medication, gentle exercise, massage, etc. (see other parts of this website for details) and most importantly a realistic approach but an optimistic one.
I hope you and your sister find the website helpful. Try not to get too down. I've had arach. for many years and I still manage a good life despite it!
All the best,
DocSarah