I think you already know the answer to this question. You have documented evidence. I suggest that your next step is to contact a lawyer about how to get the loan documents you need, and how to disengage from this mess. Possibly you could find a professional mediator.
Jupiter (brother) is in the 8th house of money that you share with other people. Jupiter has no essential dignity. He's not in a strong financial position. His wife (symbolized by Mercury) has basically become your enemy, yet she is hurting emotionally over this problem, as well.
Venus (your significator) is in a difficult position between Mars and Saturn (your father) called besiegement. You are in some ways the strongest member of your family, as Venus is exalted by Jupiter. I don't know if it's possible for you and your brother to reach some agreement, that you could subsequently discuss with the rest of your family, but basically your brother seems to feel better about you than your other family members.
As Venus applying to a conjunction with Saturn, your particular concern seems to be about helping your father through his financial issues (2nd house from the 4th) so now is a good time to stop the craziness and fix as much as you can.
Good luck with this!