Do non autistic siblings of autism kids mirror symptoms of autism?

Possibly. Newer studies show that ~ 30% of younger siblings of a child with autism display autistic behaviors early on. Of these ~ 40% are eventually diagnosed with autism, but many of the others have developmental language disorders. It is worth getting the little one evaluated & into early intervention services to re-direct that developmental path as much as possible.
Yes. Sibs will mimic behaviors frequently, especially at younger ages. If sib's symptoms are persistent or get in the way of daily functioning, seek evaluation.
Yes they can. It can be complicated as kids like copy cat behavior so it is hard to demarcate and depends on the age it is best to take the child to your paeds doctor and take discuss as it runs in family and may be just having fears of having other kids having soft signs may rule it out also great resource is yale developmental center and fred volk is leading authority our foundation is doing a lot of work.