You sister would be wise to meet with two or three different kinds of mortgage lending professionals and ask their opinion about what she can afford and what she can qualify for (which is not always the same). She may want to talk to a local bank or credit union, a mortgage broker, and a mortgage banker (these are companies which lend their own money as well as shopping the market for other loan programs). It may also be prudent, depending the interest rate for which she qualifies, for her to save some of her cash reserves and finance a larger percentage of the purchase. They will all give her those options. I do not sell property in the Beaverton area, but I do work with several metro-area lenders and can provide you with information about some reputable lenders of different types upon your request.
Is that $1,300 monthly income net or gross?
What is her credit history?
What is the price of the home she plans to buy?
How long has she been at her current occupation?
How long has she been in her current field?
Does she have documentation to verify her income?
What other debts and monthly payments does she have?
What is the source of her down payment? Is this source verifiable?
The best place to start is by talking to a loan officer. You can get tips for screening loan officers at: http://www.bybryson.com/advice_center/tips_mortgages.htm
If you need referrals to a loan officer, please feel free to contact me.
Let me recommend a couple of mortgage brokers who can sit down with the two of you and run real numbers, then let's go look at 4-5 homes in that price range.