I drove a 1989 E30 325i for 18 years and 256,000 miles. I replaced it with a 2006 E90 325i and now have 200,000 on it. The E90 is a much nicer car to drive. The E30 is now a classic, but it has it's limitations.
However, your Sister's car is a non-sport package with automatic, so you need to consider that in your decision.