It's a poor translation. Santet is like black magic or witchcraft. And in this context, 'send' is probably better replaced with 'cast'. I think it would be better to say 'casting black magic is a sin.'
Maybe voodoo is more familiar with the Western culture? But 'santet' does not need a doll or something similar. It can be send just through the 'wind' with some requirements such as strands of hair, clippings of nails, and a picture of someone you want to send this 'santet' to.
P/S: You will understand it better if you watch Indonesian horror movies :)