20 month old not talking or responding?

Basically my brother's son is 20 months old and my daughter is 19 months old. His son has always seemed different than my daughter but i know all kids are different.

But he doesn't speak words at all which i can easily say late talker because some are but he doesn't even babble or baby talk. A couple months ago they explained that their pediatrician recommended speech therapist and explained their son was delayed in speech and social skills.

So for the first time i babysitted him last month. My daughter was so excited and tried giving hugs and talking to him and just trying to get to know him and play together. When he looks at you its like hes looking through you, he doesn't respond either when you speak to him or show him stuff.

My brother and his wife don't mention anything about it even though its very obvious but i don't pry and bother them because their the type to feel offended.

When i think hes thirsty i ask him if he wants his sippy cup. He doesnt even notice or look. But if he sees the cup he'll grab it as if its cuz he knows its something he drinks from. Hes smart and knows how to play with toys ALONE but then hes randomly hitting the TV and only makes sounds and yells, doesnt respond physically when you say his name and he barely notices you around.

My brother and his wife just sit him on his tablet all day where he watches YouTube kids and that's it. He'll turn into a zombie for hours that way.

I felt bad because my daughter kept looking to us confused and seemed rejected because he wasnt responding to getting a hug and just running around and never stopping. He would grab things and we'd smile and name them for him but once again no verbal or physical response. Its like hes in his own world. 24/7.

I dont bother mentioning it to my brother because like i said their the type to be offended. But i was just wondering if anyone elses kids were like this normally or if they found out they had some sorta developmental delay or something, it was just so surprising to see.