OP, if you had a daughter and she had recieved the said Rishta, wouldn't you want to know EVERYTHING about the guy and THEN make an informed decison with your daughter?
This all depends on the extent of his drinking. If he's an excessive drinker, then it will be a very hard marriage between the three of them; your sister, the Husband and the bottle..
Yes, some people do change, but they have to want to change. Is there anything about this person that suggests he wants to change or is even able to change? If not, I wouldn't touch him with a barge-pole...