make i yearn u fact. Ure right to be concerned about ur sister. but ure wrong to dismiss someone as no get future i assume u mean in terms of material possession.  Alas wat ure looking for is not wat ur sister might be looking for to some people hapiness of heart is more important.  having said that, if ur family is the type wey no go gree am marry the guy u have to talk to her and let her know her choices. 1.  if she goes ahead she risks a hard life which usually takes its toll eventually on any marriage.  2.  She cud put the guy on hold till he is able to advance she has the opportunity to influence him at this point using marriage as a leverage.
otherwise you can just consider assasinating this guy in the name of saving the family face. just make sure ure not caught.  Do ur best and leave the rest.