Well, I know a lot of girls who are smaller DO start showing sooner, but if you are heavier it can take longer. BUT that's only ONE way of telling. Like I've knows people who you can hardly tell until about 5 months or later but I know someone who was showing quite a bit by the 3rd month.
It is probably a reassurance that your parents can help financially, at least this will make it possible for you to stay in school because you won't have to worry about how to deal with finances. You can keep talking to your sister, but a lot of towns/cities have parenting classes that I think can specifically help young moms, especially so that you can find a way to stay in school because your education is so much more important now that you are in this position.
My mom in an obstetrician so I know quite a fair bit, and I have friends who had babies quote young so if you wanna talk
PM me ok
