The problem is not that my son might turn into a bully, it is that he is so gentle that he won't fight back and will thus be an easy target for bullies.

And my daughter is just being a normal NT kid, who needs to learn to share with her brother and not thwack him every time she wants something.

It is very rare to see 2 kids love each other as much as my kids love each other. If they didn't squabble occasionally I would be more worried.

My big sister (2 years older than me) used to put me inside a car tyre when I was about 2 years old.... and roll me down a hill. The ride down was unpleasant, but the crash at the bottom was hell. I was non-verbal as well so I couldn't tell on her. And we both survived those days without child psychologists.