Bible Study Suggestion for Adult at Teen Level of Understanding?

Hi Kathy! One option to check out would be the Together curriculum from Friendship Ministries. It's designed for people with varying levels of cognitive challenges, and is totally online. I'm not certain how it would work without a facilitator, but perhaps someone at Friendship Ministries could help with that. Website is https://togethersmallgroups.org/. I'd also recommend taking a look at the free videos from The Bible Project (https://bibleproject.com/explore/)--they are fabulous, and your sister could definitely watch those on her own as a way to learn more about God's story!

I have used the video, The Gospel According to Matthew, with a middle school students. They were very engaged and touched by it. You can find it on you tube. 

Unfortunately she does not have a computer and online access.  

Kathy, if leaving home is an option, she might really enjoy being part of a Friendship group if there is one near you. Here's the form to fill out to see if there's a local group: https://friendship.org/find-a-group/