Your mother is an alcoholic. Not because her sister died. Not because her children have their own adult lives.
She is an alcoholic because she has the gene for addiction. No gene, no addiction. Every drink she ever had in her life put her at risk. Every glass of wine or champagne before you were born. Every eggnog with rum at Christmas. If someone has the gene for addiction (according to Dr. Drew) then that person is at risk with every intake of alcohol or narcotic.
Children do not know their parents' true lives. They think they do. But there is much they do not know. Many parents drink after their children are in bed and the children do not witness the drinking. Alcoholism can have a long, mild early stage which can last decades. Then it progresses to the middle stage. Sometimes a life change accelerates the progression to the middle stage and someone who seemed to have no drinking problems suddenly has them, visible to all. But in reality, the addiction was developing for years. If you read the book "The Addictive Personality", you will learn about these stages of the disease.
Alcoholics love alcohol, and they will not give it up until a series of painful consequences become so overwhelming to them, and they are so desperate to escape the consequences of drinking, that they enter recovery, the love affair with alcohol having become no longer a comfort but a nightmare.
Your mother will manipulate you so to divert your attention from the facts: she is an alcoholic and that, and that alone, is the reason she drinks.
She has grief, life issues, relationship issues, unhealthy patterns, of course, which she would have to address in sobriety. The less stress, the healthier the psyche, the more solid the sobriety.
But she drinks because she has addictive disease. And nothing you do or say will ever touch that.
So put a safe emotional distance between you and her--get a counselor if necessary, to process the irrational guilt and inevitable anger--and find your way to acceptance that you did not cause her alcoholism, cannot control it, and will never cure it.
Wishing you loving detachment from a life which belongs only to her, and is not now and never has been your responsibility.