I'm 20 and can't even imagine being married right now. I have been with my guy for 4yrs, but we have been best friends since we were 10. We have talked about it, but I told him I was not going to do that until we both graduate college, gets jobs, and can actually support ourselves. So I'm (hopefully)planning to be 26 or 27 before I get married.
My stepsister married a guy at 18. He was in the army, top of the class. Was going to be an engineer. He is the very jealous type and proposed to her about a month before basic training. They got married the day before he left, and she moved with him. She did not go to school, and just stayed at home. He eventually quit basic training, came back up, and they have now lived with his parents for 4 yrs. She still has no drivers license, and works at a gas station.
Just goes to show that not all young people are suited to be married. It sounds like your sister is different and I hop they have a good life together.