You may have seen a few hundred IMS failure posts on the internet (out of 200k+ Boxsters), but how many where for 1998 Boxsters? I have only ever seen one.
No one likes the potential for a $10k engine replacement, but few people take the time to realize what a low percentage of Boxsters it happens too. Just like many other potential Boxster owners you need to realize there are a disproportionate number of the people complaining on the internet vs. people praising how few problems they have.
I can only tell you what I know (which I like to think is quite a bit since I have been reading and learning about the Boxster for over three years). It's up to you to decide whether you want to believe me or not, and worry about issues that don't affect a 1998 Boxster.
If your sister has taken of the car, and it is in good condition with those miles I would jump on it. But I would still get a PPI (even though it is your sister) because cars that sit a lot tend to have some minor problems like vacuum leaks and bad hoses.
Summary:
Slipped Sleeve Failure - Affected 10/1/98-3/31/99 Check your door sticker for the date, and if you fall into those dates check to see if the engine has been replaced. If it hasn't failed in the first 10k miles it most likely is not going to fail.
IMS - Extremely rarely affects pre-2001 cars (same likelihood as any other car made today). The IMS was redesigned for MY2001 to be quieter however the new design, while quieter had some design defects that allowed the IMS to fail without warning.
RMS - Affects all 986 Boxsters, due to imperfections in the casting of the engine. A slow leak forms which can be fixed. If it is not fixed all of the oil can drain from the engine suddenly causing engine failure.
Edit - I have no intention of sounding rude or mean or short, but you are not the first potential owner that has posted about how they are hesitant about buying a Boxster due to the engine failure issues. Almost all of the potential owners that post on this forum bring this up, and it is tiring because most of them are not well informed. I am not saying an engine failure will not happen, but I am saying it isn't very likely.