Is your BIL a reasonable man? Does he have both the common sense and compassion that would respond to the honesty you've just shared here? If so, tell him that you've waited until the right words might come but they rarely present themselves easily... the bottom line is that safety might be compromised as a result of work done which doesn't incorporate all the necessary safety measures. When safety is compromised there is a burden of liability and the possibility of regret that you'd rather him not have to face.
