My sister is pregnant?

So I'm going to be married in a few weeks and my fianc� (and gf of 6 yrs) is 3 months pregnant and everyone happy for us. However, my 12 year old sister told my mom last night she was 4 months pregnant with the baby of an 18 yr old boy from a poor neighborhood. We have a lot of money and he said she paid him 200 bucks for sex and he accepted because his parents needed the money. Now he wants nothing to do with the baby. She is getting quite big and my fianc� doesn't want her at our wedding because she said it will shame our family. Probably right. She's the youngest of 5 children and only girl. My mom is furious, but my sister wont listen to a word she says. What should we do?