I bought my car in 07 w/ 12k miles on it...the tires were semi-decent when I bought it...they are the Conti touring as well...I have 33.5k miles now...I replaced them a few thousand miles ago with Bridgestone's listed in my sig...I never drove the Conti's in the winter either...I have Hankook winter Ipike tires...my guess is the tires had a total of 22-25k miles on them when replaced with new tires...they weren't completely gone, but I never drove them in the winter and hated driving in rain with them (even dry pavement sucked)...they are horrid tires and I can't believe they are considered all seasons.
I also had 2 nails in one tire within a couple months of buying the car and just put plugs in them and drove on it for 2 years...don't recommend that, but whatever. I'm extremely happy with the Potenza's though.
As for the OP...it sounds like your sis' car has aftermarket wheels? Since you talk about low profile tires...are the tires stock or not?
Check this link: Tire Size Calculator - tire & wheel plus sizing
You can input the stock size (or look up the car)...the stock size for 15" tires is 195/60 15 and for 16" it's 205/55 16. If the car 18's stock (SS/SC), I don't know what stock size is. It wouldn't have 17's would it? They didn't make an SS/NA in 05 did they?
Anyway, check your stock size against what you have now (if it's aftermarket) to see what the difference is or play with the numbers to see what you can do.
Here's a quick reference of what is generally used for each:
15": 195/60, 205/55, 205/60
16": 205/55, 225/50
18": 225/40, 215/40, 215/45, 225/45. 235/40
Hope this helps somewhat.