This question has been quite old though many other buyers may also be looking for a better answer on this. Being in Real Estate and Lending since last many years I have come across many such scenarios in which a co-signer is sought.
One must understand the difference between Co-borrower and Co-signer first.
Co-borrower- is one who buys the property with you, is on the purchase contract, will be on the mortgage note and on the Deed/Title of the house
Co-signer- on the contrary is used for the purpose of qualifying on the mortgage only- it may be due to low credit score, and/or high DTI ratio. When one uses co-signer he/she should qualify for the PITIA on the property by herself/himself but for the DTI can use any close relative. This co-signer does not have to be on the title as he/she is only taking responsibility for mortgage end in case of default.
Freddie Mac and Fannie Mae have very clearly defined them see link below
page 16 cosigner.
However, problem is many lenders and Underwriters do not differentiate between co-borrower and co-signer or they do not want to. Same is the case with many attorney's. They have to be shown and educated about it.
I have lent on many such cases.