Family issue, don't know what to do to solve this

My sister 27 is in love with a guy who is 24. He completed school but didn't go to college, working in a provisional shop in Chennai for 15k salary. The problem is he used to have a drinking problem and doesn't get along well with his elder brothers. He is a distant relative, and him and my sister are cousin brother and sister. His mother and few of his family members lives in the same village as us. And they are okay with their love. Everyone in our family is against this. My sister has completed M.com and preparing for TNPSC, I don't know what she sees in him. We are not looking status here, first I don't like they are cousins, and he doesn't have a good past. She says he is changed, but there is no guarantee he won't start drinking again. The disparity in their education and income if she gets a job will be high, it will cause a lot of problems. My dad passed away this year march, he didn't want this marriage to happen too. She has rejected all alliances citing her TNPSC preparation, I think she only wants to do love marriage that's why she rejected all the alliance proposals. She also gives her health as a reason to not want to do arrange marriage, she has epilepsy. She had 3 love failures before, and her second love didn't end in marriage because of her epilepsy. Maybe that made her feel, insecure and thought he or his family won't say no to her because she has a college degree. She says that she decided to marry him after talking to him for just 3 months, I think she made this decision without thinking much about it and want to settle for someone who wouldn't say no. It's been a year since we knew about this. Tried to convince her she won't have good future with that guy, but she is not listening. She says if she can't marry him, she will stay unmarried forever. We don't know what to do to convince her she is making a big mistake.