Since you are the groom's sister, wear it on the Walema. I think its really cute when sisters or cousins or sister in laws wear their own bridal dresses on someone's wedding. Since you are saying its not even that heavy then go for it. As long as you don't do the whole dupatta over the head thing you should be fine. You can even wear your whole bridal set.
As Khawateen mentioned there is no way any other woman can steal the bride's thunder.