Sister's on and off again boyfriend

My sister and her boyfriend break up then get back together over and over again. They just broke up again today and he left with all of the belongings from their apartment. My final numbers are absolutely due by Wednesday along with a finalized seating chart. My family and I aren't fans of the guy because he is a liar. He has lied to me, my father, my mom and my sister. Once he gets caught then he admits the truth. He has also encouraged bad choices. The other night he was encouraging her to do drugs. He was a recovering addict, but it seems he has fallen off the wagon and has gotten my sister involved. Prior to him my sister was sleeping around so I don't want some random at the wedding that we are never going to see again. I also don't think she needs to go back to this boyfriend. On top of everything else, they fight constantly and he has a young child who shouldn't be subject to this. I want my sister to be happy and she tells us she isn't happy with him and she breaks up with him, but yet when they are broke up she cries because he is gone. I don't know how to approach all of this. On the one hand, I know if we take him off the guest list she will be upset if they get back together, but on the other I am not willing spend $80+ on him. He haven't decided if we are upgrading chairs but if we do they are an $8.50 per person so it would be closer to $90 for someone who might not attend.