This is just an amazing coincidence! I'm through an exact situation although my sister is 30 and has been married for 7 years, now divorced her husband which was a very nice guy and moved to parents house. Now she dates a total ass who's 31, who never had a job in his life and causes all sort of emotional trouble to my sister and indirectly to our family, still she won't leave him, although she also said she would. Our family also dissaproves it greatly.
I bet it's genetic on women to simply leave the group... just because this one has better looks than the precendent. We just decided to leave it to herself to decide and be responsible for her acts!
It pains me greatly as a brother to see my sister be dragged to the depths of self-esteem like this and drowned in bank debts. But this all due to her decisions.
My advice is to let it go, just drop it. She will learn, eventually, and be responsible for our acts is what makes up adults.
Good luck!
"E ao imenso e possível oceano
Ensinam estas Quinas, que aqui vês,
Que o mar com fim será grego ou romano:
O mar sem fim é português."