There is much involved because anoxia can damage different part of the brain. If this is your twin sister, she must be an adult. The best indications of her mental standing would be her school records. If she has seizures, you need to get her medical records and talk to a doctor about treatment plans. You may want your sister to go to a day program like a workshop. However your sister has been is a good indication of how she will be. Don't be afraid to ask for help but do be prepared to advocate for her.