I did use the India tag as a matter of course, but as always welcome answers from members all over the world. Why? Because some of the most helpful answers to my previous India-tagged questions came from non-Indian members here at IPS.SE  

Note: Two of my cousins and one friend have already benefited from the good advice given by members of IPS.SE but I was not directly involved in those cases whereas I would be if I am to convince my cousin.

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Cultural Tradition
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In India, cousins are often called 'cousin brother' or 'cousin sister' to indicate gender and also closeness. Our family is very large which is why I have been asking questions about many different relatives here. Why? Because we have some measure of 'social responsibility' to help our relations in trouble.

Problem
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One of my cousin sisters has decided to marry a person whom I personally know to have 'bad habits' and is unreliable. I should not usually intervene in such matters, especially when my cousin has actually selected this person herself, but her parents are anxious because this is a local person of known bad character and unlikely to improve because he is a habitual offender. 

But she flatly refuses to believe them that he will not improve and she is now in a 'big fight' with them over this issue, so they have requested me to use my influence (and also family responsibility as a 'cousin brother') to make her understand the danger. Is there any diplomatic and effective way to do so? More details follow.

Family History
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My cousin sister [code name = Little Bun which is actually her pet name kept by her mother] is 24 years old and well-employed in a major company in a big city in India, far from our home city. When she visited her parents here recently she met a local man, aged 28 [code name = Joe], who has charmed her so thoroughly that she took a very quick decision to marry him just 6 weeks after their first meeting. They have fixed their marriage for October 12th and both sets of parents are invited, yes, but they have been cuttingly advised not to take any steps to stop this very small-scale wedding.

“Little Bun” the bride-to-be
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Specifically, I am worried because Little Bun is making a big salary and Joe needs the money. Moreover, she is the only child of wealthy parents who own valuable properties in this city. It is sad but true that such girls have been targets of unscrupulous marriages in this region. I have seen more than one such local marriage end in cheating and bitter divorce, so I hesitate to tell her parents that I cannot help them in this delicate matter. However, I am absolutely not going to communicate with Joe in any manner over this or any other issue.

“Joe” the groom-to-be
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The problem: Joe is notorious in this town as an unreliable character with plenty of 'bad habits' (read vices) -- this is no exaggeration because I know for a fact that he drinks alcohol in excess, gambles away his earnings and is being investigated for fraud at his former workplace. He used to be a low-level manager in a local firm but nowadays operates 2 'tourist taxis' which run to faraway towns and give him a good excuse to leave the city when he needs to. 

There is no caste- or religion- or socio-economic differences causing her parents to oppose this marriage. It's purely the concern about the known character of this Joe. However, Little Bun, who was always treated as a child by her parents, is in rebellion mode and refuses to hear anything against her fiance.

In typical movie style, Joe has convinced her that he is the 'victim of circumstances and social conspiracies' that refuse to give him a chance to lift himself up and away from his 'unfortunate past.' As in, she is not exactly unaware of his reputation but he has given it a romantic twist and also misrepresented the facts to his advantage.

Myself
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I am rather influential with Little Bun who is 14 years younger and good at heart. But if I intercede on behalf of her parents I might possibly not convince her, but also risk losing that influence. On the other hand, I am very worried that Joe will lead her into big trouble because although I don't know him very well myself, his past history is well known locally and he has even been entangled with a few girls leading to much complications and heartbreak. 

What I have tried
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I have not discussed the matter with her yet, simply because I can't do it more than once and want to prepare the best approach.

So here I am at IPS.SE:

Is it possible for me to diplomatically convince my 'cousin sister' that she is going to marry a person with 'bad habits?'