7270 is actually a boy...i mean, a male. LOL
here's one thing you can do...
you could tell her that you respect her decisions, and that although it is strange to think that she is all "grown up" now, you worry for her (as the big sister). butter her up so that you could get information out of her. support leads to trust...you have to be there for your sister (even if it means she has to learn a harsh lesson). once she feels comfortable with your status on the matter, she is more than likely to open up to you. in this stage, you will be able to ask her all sorts of questions in which the answers may be more genuine than if you were to disapprove and interrogate her actions. despite how you feel, you don't want to completely close up to the possibility that they might end up together no matter what. what's really important is that she answers to her own circumstance. for example, instead of asserting your opinions, ask her (casually...like in a girl talk or something...here and there) to describe all the things she loves about him, and how she plans to make it work out in the future. even ask her to describe all the things he's done for her, and what things he has promised to do in the future. from these answers, you will be able to identify how mature this relationship really is. if she doesn't have concrete and/or specific details, then she is probably confused...unless by nature, she is already a creature of very little details to be said.
haha this is way too early for me. slept late. hope everything i said makes sense.