when you get together to meet him next week, and the topic of rehab is discussed and if he agrees to it, make sure he completes rehab before any significant steps are taken towards a wedding. Work on compiling a list of questions for him, and I'm sure the guppies here will be able to help...
I personally dont think she should get married to him since he probably wont change (I have seen the effects of alcohol first hand). But you need to tell your parents, if you want whats best for your sister. And read isthikara.
your sister says she loves his family, etc... so at least she'll have someone to cry to when she discovers what its really like to be around someone that drinks and probably sleeps around. What is she doing to do when her in-laws are no longer around and have passed away?