Well, I agree with Marm, that working things through is the mature thing to do. In absense of physical and/or emotional abuse...
I don't get the boyfriend? I mean where are his balls? Where are his brains?
Where is his own self-respect?
What about the girlfriend? Sounds like it's been a long time. Has she talked with and/or confronted him about how she feels? Or is she afraid to?
Why is that even a question.
There's a big difference between reasonably working something through. And spotting an unreasonable amount of dysfunction...
If I were the boyfriend? The situation wouldn't even exist. My girlfriend wouldn't be left to ask strangers how to handle such a scenario. Because it'd already been addressed.
The way I see it...the problem isn't with the ex- it's not with the sister, it's not with the family. It's with the boyfriend.