Thanks for all the replies thusfar. Really appreiciate them all. As of background, my sister and I have been living together for over 5 years so I know her situation very well and am not guessing on her life. I know most of the friends she sees (has 4 close friends)and does. I do admit that she probably has been on dates that I may not know about, but am sure they are few, if any.
In terms of online dating, I got her an annual membership to match which she refused to use for some reason of not liking those types of sites. She just is not comfortable meeting men that way. I do agree maybe some form of depression is prohibiting her from truly being outgoing, but she claims that it is depression which has been much improved as of late (since she found a new job in the past year) and that it is much more controllable today (even though she does admit that even today, it is slightly above the norm in terms of the depression swings she gets compared to other women). I do beleive that her depression is much improved when she was out of work for several years, but still not sure if it is improved enough, but she refuses to see a therapist.
In terms of the introvertedness, yes it is probably something one cannot change, even though that personality trait is not set in stone (i.e. I have shifted from a borderline introvert to borderline extrovert to deep extrovert in my 20's to 30's). However, introvertedness does not prohibit women from meeting new people and should not be a major deterent in starting and maintaining a healthy relationship with the opposite sex (even though I admit it is tougher for introverts to meet new people, but many introverts which constitutes roughly 50% of the world's population have healthy relationships).
I think net-net, it has been a while since she has actively dated and I want to help her get jump started. 2-3 years ago she thought it would be impossible for her to get a job after being out of work for several years with not such a stellar resume and after helping her out, she got a great finance job at one of the top investment banks. Similarly, she believes that it would be impossible for her to get married at this point and I beg to differ that this is no different than how she felt three years ago that it would be impossible to get a decent job. I feel if she sets her mind to it, it really shouldn't be so hard and she really hasn't even tried. A little confused on why simple life issues are so tough for her. I know she prefers to get married, but she rarely goes out and is not "in the market" to even have a chance. Any thoughts on how to get her jump started or anything relevant that you would help would be appreciated. Thanks, guys (and gals?).