Ridiculous Guy
While I probably wouldn't like it, I would trust my sister's judgement but also tactfully let her know I think she's making the wrong choice then try to get to know the person she wanted to be with now to see if they were worthy of putting my trust in them in regards to my sister. Regardless, it is her choice to make on who she wishes to be with, yes?
I'm sure you don't like it and probably feel betrayed, but it is her life to live, not yours. Would you rather see her happy with someone she actually cares about and have it last than to be with someone she will probably be miserable with?
At the end of the day, it is her decision to make on who she wants to be with. The best we brothers can do is just be there for them through the happy times as well as the bad.