Should I break up a couple? PLEASE PLEASE HELP!?

Ok, my sister is going out with a guy who is no good for her (illegal, childish, conceited, just plainly a dumb!) my family all knows he's bad for her, I love my sister to death but she doesn't listen when I talk to her! I am thinking of sort of setting her up on a date with one of my college buddies (us hanging out) so she can ditch the loser she wants to marry. Get this! She wants to run away with him to the poorest part of Mexico and his dumb already wants a kid even though they're so young! supposedly she can handle it but I know she wont last more than a month! What should I do!?! If it matters they've been together for like 1-2 months (I know right?) And I'm 19 she's 18, and that jack is 19 were both legal mexican/americans, he's not. she f*cken buys him expensive *** clothes! While he gets her cheap sh!t from target. My girlfriend says he's not even cute, he looks like a stoned kermit the frog. Please HELP!