Ha ha, youtube and music - your sister’s interests sound similar to mine!!
I’m not London based, but have you tried asking at your local sibling group? There will be others there who have a brother or sister with learning disabilities and they may know of other local services for your sister (note - this a group for you, rather than your sister). Visit https://www.sibs.org.uk/support-for-adult-siblings/adult-sibling-support-groups/ to sign up.
Try searching on google for local mencap branches or call mencap’s learning disability helpline (0808 808 1111) who can put you in touch with your nearest branch.
Keep in touch and let us know how it goes!