Thank you for your question and welcome to Just Answer. I am sorry to hear of your sister's problem.
The connection between alcohol abuse and dental disease is twofold.
1. Alcohol is metabolized by the bacteria in the mouth into sugars. The bacteria then metabolize the sugars into an acid, which dissolves the enamel of the tooth and starts dental decay. The constant bathing of the teeth in alcohol throughout the day accelerated the process, whereas in a normal diet, sugars and alcohol might be ingested during mealtime only. Periodontal disease is also prevalent in alcoholics. See:
Scientific studies are cited on this webpage to support the claims.
2. The alcoholic usually does not maintain good oral hygiene habits, slacking off on brushing and flossing, which leaves the decay causing bacteria on the teeth for days at a time and compounding the problem.
In addition, heavy alcohol consumption increases the risk for developing cancer.
I hope you find this information helpful. If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
George McKee DDS