Generally all the income from your parents and your wife can be combined. The tax returns for your parents will be reviewed for the last two years to calculate the investment and any rental income.If the combined debt will be divided by the combined income to calculate a blended debt ratio. If that is enough to qualify you should be okay. Many lenders will tell you that this type of loan can only be done through the FHA but you can go conventional if your lender works with Freddie Mac. I agree with the other advisers here. This situation is way too complicated to do on your own. Talk to a Loan Officer in your area.-Mike
In my opinion, an online lender may be okay for a refinance but you should always use a local lender for a purchase. Your Realtor can help recommend someone.
I would start the pre-approval process now and see if your wife can qualify. If so and you can use her income, you can be more aggressive when you prepare your taxes. Depending on your time frame for buying, if she is a near miss on credit, she can start working on her credit to increase her score.If she cannot qualify, you will know how much income you will need to qualify.
Sometime in March, local banks will offer the Welcome Home program. Limited funds are available so it goes quickly. Best program out there if you qualify and it's withing your time frame to buy.