Temporary visa for partents needs some advice

Hello everyone,

My dad and mom wants to come to canada to visit. i have already sponsor for them 2 years ago still no progress. My wife is 5 months pregnant. this is our first baby.she is a canadian citizen and i am permanent resident in canana applied for passport. my wife's family is here. i will send them letter from hospital that my wife is pregant, our notice of assessment $15000 we didnt work last year, my current job letter 40 hrs and my wifes job letter 25 hrs week since last August 2008. i dont have enough money on bank acct app $2500 as i have to pay bills and everything. but my parents have app $75,000 at bank and house and car and few assets to show. and my younger brother is staying there. my question is

what are the chances to get temp visa for 6 months?
should i get letter directly from doctor that she needs some assistance or general letter from hospital? does is make any difference?
when is the best time to apply for them? my wife's due date is on december?
Is there anything else we can show that will help us to get them visa?

Your advice will be highly appreciated..