You seem to be on the right track. Is there a color scheme for the wedding (Like, what colors are any bridesmaids wearing?). That may help with the color choice for shirt and tie. Vests are fine. If it makes you feel any better, every eye will be on the bride (and groom). All you need to do is be respectable -- you will be, your choices are appropriate. Enjoy yourself !!

It dpends on what type of wedding it is,, is it in a church, or very formal venue? Or a registry office? Is your sister wearing the big wedding dress, or a less formal one? All this would tell us just how formal a do this is, and that would tell us what you should wear. If it isn't very formal, then black trousers and shoes, with your blue shirt should be fine. Add a tie if you want, maybe a dark blue one, and don;t bother with the vest.. it would just look odd without a jacket, and make you look like a waiter

yes, since you don't have a jacket or can't afford one at least a vest, nice shirt and tie will be appropriate for a wedding