Unless your in-laws are trained in the treatment of autism, they can't help much. And maybe he has a bad attitude, BUT sensory issues are HUGE for autistic children - he may not just "hate" brushing and bathing for "no reason", they may be physically uncomfortable to him. Would you like to brush your teeth if it felt like sand paper on your gums? That may be his physical experience.
Contact Autismspeaks.org and see if they can recommend resources for you. Discipline for an autistic child is fine, but their brains do NOT work like ours - think of the difference between Mac and Windows computers - they both work, BUT they don't work the same WAY. Both of you need to understand what is going on with him and work with that. This doesn't mean NOT disciplining, it means disciplining in a way that will work with him, and it probably won't be quite the same as what you or your husband grew up with. So you may end up telling family "thanks, but we've got it" in terms of their "input".
Spankings to "teach" your older child not to hurt your younger is not logical and will not make sense to him.
You need some professional help to deal with this to find the best ways to communicate with him AND help him communicate with you and process his world.
My son is very high up on the scale - when he has an outburst, he does the "NO" thing and you can't talk to him till he's processed everything and calmed down. THEN we can talk. Don't know if your son is the same, but you need to get help to figure out how his brain is working.