In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Answering your question, Sheikh Mustafa Umar, President of California Islamic University, states:
The illness of autism does not eliminate sexual desires from a person. Therefore, since a cousin is someone who is permissible to marry in Islam, your wife must wear the hijab in front of her autistic cousin.
Almighty Allah knows best.