Hi MotherFunk,
Your local Contact a Family may have information about social groups suitable for your sister.
Also contact your local council and ask if there is a local disabilities forum. There may be a member who is aware of a local group.
Another useful contact may also be your local councillor. If there is a local group they probably apply for funding from the local council and so they may be aware of it. If not they will have relevant contacts in the council and community to find out more for you.
Another option for you is to contact a local secondary special needs school if you have one. They would probably be aware of local provision as they would hopefully be referring students to them as they leave.
Hope this helps.