This is actually pretty common with Jack Russells. As puppies, they tend to be pretty full of themselves and need some consistent training. Your sister, and every human in the household needs to establish themselves as higher ranking than Jake in the pack. They should make Jake work for everything (ie: he needs to sit before he can go outside, lay down before he's fed, sit before he's pet, etc etc etc). They shouldn't allow him to get up on the furniture at all until this problem is under control. This is really something that your sister should see a behaviorist for. The behaviorist should be able to tell your sister several helpful things with just one session. Then she can decide if she wants to do another session or not.

You might want to show these links to your sister:

http://www.ddfl.org/behavior/nilif.htm

http://home.achilles.net/~madigan/Raising2.htm

The second link especially would be good for her to read.