Hi there illilady!
I just wanted to say that your nephew is lucky to have such a caring auntie.
With regards to how best to communicate with your nephew; autistic children, and oother children with communication disorders often have trouble processing voices and other aural communication.
You need to make sure you speak slowly and clearly, and keep messages short and to the point. As few words in your sentences as possible. If he doesn't understand, give him time, and repeat what you said in exactly the same words. Give him a chance to process the message, and make sure you don't change it so he has to start again.
Also just ask him to do one thing at a time, and be very specific.
For example if I want my 6 year old autistic son to pass me the telephone, I will say "Luke pick up the phone", then "Luke give the phone to Mummy", instead of "Pass me the phone, please". Try not to use "I, you, me, etc" and use his name and your name - autistic children often have great difficulty with pronouns.
Also use visual cues as much as possible; often autistic children are much stronger visual processors than anything else. You don't have to know sign language , just use as many gestures as you can, eg/ mime drinking from a cup when asking him if he wants a drink, put your hands downwards as you ask him to sit down, put a hand up flat when asking him to stop. If you are trying to explain something doing little very simple drawings can help, ( and I mean simple, like stick men, you don't have to be an artist!).
Another thing it's worth remembering is that often these children have difficulty understanding long explanations of why they should or shouldn't do things. With my son I got most success just by telling him something was "Not for touching" or "Not for playing with", or just "No -------" (insert activity you want him to stop, eg/ biting, kicking, hitting etc). He would accept this instruction as adequate, whereas my other son with no special needs would always need a full explanation or he just asks endlessly "why?"..
The main thing I think you need is patience, but it is so rewarding if you do get a positive response. It can be so frustrating trying to parent these children, and you are right when you say that physical punishment is counterproductive. There are some things which they will just find too hard to do, that you may expect a child of their age to be able to, such as sitting at a table for a meal. The harder you push the harder it becomes for them. They need to take little, sometimes tiny steps. Remember that many social situations are terrifying for them. They need much encouragement and you sometimes have to let them take it at their own pace. This isn't to say they won't get there eventually.
Sorry to go on for so long, I got a bit carried away. Hope some of this helps, and good luck with your SIL. It's very hard on you but remember however upset she may be with you initially she will thank you in the end. I think your plan of using the TV programme ruse is a good one; well it worked on me when a close friend first alerted me to the "A" word...
All the best, Leise