If your brother is not interested in proceeding with the proposed marriage, he can very well break this engagement and cancel the proposal for marriage either by oral or by a communication in writing .
He may state some reason for this decision and in case he apprehends some problem, he may indicate that he would be inclined to compensate the expenses to some extent, which would pave way for an amicable settlement of this issue.
Try to pacify the bride's father to extent possible, if he is not convinced then send the communication by registered post, keep the acknowledgment card safely so that it can help tide over the legal issues at a later stage if her father decides to take up the issue legally on some false grounds.