This happened with my Sister she turned 2 and wasn’t speaking, I’m 13 years older than her and my parents didn’t do a ton of interaction with her as in sitting with the books and bricks etc. I was doing a lot though because she was obsessed with me
They couldn’t understand it because I didn’t have what she had and I was a gobshite very very early on
Anyway one day me, my Dad and her had been in the car somewhere and we got back to ours and one set of our Grandparents had just arrived to visit they lived in Spain. We walked in everybody was there greeting us my Mum said something to her and she just turned around and said ‘Daddy called Jane a
bleeping idiot in the car Mummy’. We were all in absolute shock
And yes I had been arguing with my Dad in the car I was about 15 at this point