The sign up page on the Sibs website is at the following link: http://bit.ly/1oHzzA5
There are other support groups throughout the country.
Here is a link to a local news article about the group: https://ealing.nub.news/n/west-london-adult-sibling-support-group-to-launch-virtually-next-month-supporting-those-who-have-siblings-with-disabilities