Alcohol has very bad Affect both on Teenager and the relationship with their Family members. When Teenagers are heavily drunk they lose their senses and become more aggressive and don't obey their elders. And in later stages Alcohol have bad effect the health of an individual and even lead to stomach cancer.
If the parents of the teens drink a lot around them they will get the idea that the parents don't care and that they can do whatever they want. If teens start drinking they could become addicted and later will get alcohol poisoning or do something they will regret like steeling a car or other stuff like that. If the parents want to drink they should wait till the teens are out of the house for the day. If you think the teen is drinking you need to find out and try to stop it