My twin sister is sleeping with her brother
I would like you to do a topic on how one feels to know that her identical twin sister was seen having sexual intercourse with her brother. People saw them. It is very embarrassing and disgraceful.
You should speak to them and encourage them to make an appointment to see a psychologist.
That behaviour is abnormal and abominable. You say people are aware that they have been having sex. I hope that you are not lying on them.
If your twin sister is involved with your brother in a sexual way, you should be able to speak to her. She should listen to you.
If you are sure that this is happening, your parents should be told, so that they too can encourage them to seek professional help.