I'm so sorry to hear about your difficulties! My sister is a recovering opiate addict, and I too have had to deal in the past with the frustration of watching my mother enable her. I think that in a way, the same principles of allowing an addict to deal with their own consequences applies to people who enable addicts. As you said, your Mom knew the risk she was taking. Maybe if she has to deal with the consequences, then she will learn her lesson and be more cautious about lending your brother money next time. FWIW, I don't think you should feel guilty about saying no. I think that not lending money to your Mom to make up for money she has given to your addicted brother is a very reasonable boundary.