I can quite see that she is never going to grow up while her parents are refusing to make her stand on her on two feet. And like you I would be very concerned about a middle aged man with no home and no job - he must be nearer you B in age than your niece - just sponging off someone.
I suppose that needy parents bring up independent children because they are so busy with their own lives they dont have time to molly coddle them. Likewise strong independent parents are always there for their children and taken to extremes can result in an adult who has never grown up which appears to be the problem you are dealing with.
Sometimes it is very difficult for those close to a problem to discuss it dispassionately in which case a neutral third party can sometimes help. Could you suggest to your B & SIL that they get couselling of some sort - initially just them to sort their own feelings out but evenually with your niece and the boy? man friend to make them understand the situation better.
If something isn't done soon your B could find he's got a life sentence.