If you want to replace the default Ecwid`s "Sign in" link with a custom one, perform these steps:
1. Use this custom CSS rule to remove the current "Sign in" link
div.ecwid-productBrowser-auth {
display: none;
}
2. Then add a custom link to your site, it should point to http://PATH-TO-STORE#ecwid:mode=accountSettings (replace PATH-TO-STORE with the real URL of your Ecwid store)
For example: http://www.ecwid.com/demo-frontend.html#ecwid:mode=accountSettings