Question: What can family members (excluding parents and siblings) do to help a loved one with autism?
Answer: Other family members can do two large things to help families, or parents of kids with autism. The first one, of course, is your unconditional support and regard. It's very important to avoid any notion that the parents are to blame for their child's autism and to speak up loudly if the parents get that message from any other family members.
While there may be things that all of us parents can do to improve out parenting or do things differently, autism is not cause by a parental dysfunction.
The second area of support would be forming a relationship with that child. Kids of all types need as many relationships and loving family members as they can get. And going that extra mile to work to form a relationship with that child is helpful. It can also be key for being able to care for that child to give the parents a break.
Anything from one hour of babysitting to more extended stays with a family member can be absolutely critical for that family. Being able to care for that child and have the parents trust you to do so is worth a hundred casseroles.