To me, flat out saying to her that she is a danger to herself, and a danger to your son, is the best thing for her to hear. It's very hard to walk that fine line of trying to help her in her sobriety and simultaneously not have resentments (detachment). It's sad when a parent does not see that their alcohol intake is truly impeding the right their child has to be protected. You handled it the best you could, IMO, with plain facts, even in how you described what it was like for you when she entered inpatient rehab (you did not "engage", it sounded matter-of-fact, to me).