Welcome to BOL.
Your bank's loan policy should dictate what is acceptable for borrowing, and should cover these scenarios e.g. ("we will lend where there is an acceptable co-borrower" . . . or . . . . . "a co-borrower can add credit strength where the applicant has limited credit, but not as a substitute for bad credit". . . .)
The concern might be the age of 86 and any type of elder abuse laws. I'm thinking if there are other siblings, and dad passes away, with most of his fortune tied up with one child. The others could try to implicate the bank.
Integrity. With it, nothing else matters. Without it, nothing else matters.