6.75% is not a low assumption. However you look at it, this is a subprime mortgage.
If you do the mortgage yourself, you're way overleveraged, income-wise.
If you somehow partner with your brother, they're going to look at his poor credit score.
Your wife's income, when she had it, was all unverified.
NOBODY is touching sub-prime or Alt-A mortgages today because nobody can sell them into the secondary market, there are simply no buyers out there. The credit crunch is still alive and well and all-powerful in this segment of the mortgage market.
If you DO find someone willing to fund you (with or without your brother), it ain't going to be at 5.6% Guaran-sheed.