Before you make your sister's cake, zanele, you should find out what she and her betrothed prefer and/or what they would like their guests to enjoy.
Is a big cake desired? Do they want a main bridal cake and several other small cakes or cupcakes (which seem popular these days), several lovely sheet cakes to compliment their main cake?
Then we get to the type of fillings and icings. Those can be a separate discussion on their own.
As you can see, a wedding cake is not like any other "special occasion" cake.
Once everyone's chosen the cake flavor, I would recommend that you bake several test cakes to determine if the cake turns out to everyone's acceptable level...and taste. Better to goof up before the wedding than to serve something that folks will remember as awful or mundane.
However, you don't necessarily have to shoot for fabulous, delicious and somewhat memorable will be just as good and better for your nerves.
I'm sure you want to produce the perfect cake for your sister and, with adequate and careful planning, you will provide a winner.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!