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Your problem is what is showing for taxable income after business expenses. This is a problem for a lot of people, but you simply cannot get around it. You have to show enough income that is taxable to you personally to support the loan request and your other debts.
The issue with your brother joining the business is a nonissue. Sounds like they are trying to blow you off and piling on.
I didn't think that mortgage lenders would allow for co-signors or anyone not actually living in the home and on title. I may be mistaken.
If your tax returns' two year average is 20K income, that's what will be considered, end of story. Sorry.
FHA works for W-2 type workers, not so much for self employed. Plus IME Quicken does not handle self-employed well at all so I am not surprised you have had a difficult time with them. If they had good telephone reps they could have advised you in the first 5 minutes that $20k won't support a mortgage payment. Also, big box banks are very conservative. You might be able to work with a small regional type bank, but really your best best is to make application with a correspondent lender/mortgage banker. Make sure to discuss your self employment income first with the LO to find out what programs that mortgage banker has available to the self employed borrower.
You do have an issue because if your income is showing $20k after deductions, there isn't enough income to support a mortgage using FHA standards.
However, there are loans that are readily available for your situation. The most common type is a 'bank statement' type loan. You will need to put more down than for an FHA loan, but you don't have the associated income issues.
I will pm to you a couple of excellent LO's that can help you.
I suggest you pull the 3B report here so you know your mid-mortgage score not your FICO 8 score so when you speak to the LO's you can give accurate information regarding your credit score without having to have your credit pulled until you make a lender choice.
It would be best to get the mortgage on your own and not involve your Mother if at all possible. In your discussions with either of these LO's they will be able to advise the best route to help you given your situation.