Guidance needed for supporting siblings please

My son is 4 and has just been diagnosed with autism, it wasn't unexpected. He is fully verbal and advanced in some areas but has difficulty with processing emotions, socialising, rigid thought patterns and the concept that others speak and behave different to himself. He is too young to be assessed for ADHD but has many traits of this also. 

This has a direct impact on his younger sister who is only 2. She cannot understand, and becomes very upset when her brother is unregulated. 

I am seeking advice, guidance and knowledge on how I can support her more emotionally through this process. Any strategies I can use or try also. 

She has one on one time with myself and her dad regularly as does her brother, this is very important to all of us and I am conscious that it is very much needed more so because of the struggles we face daily.