Should a 3 year old know how to behave?

My 3 year old nephew (almost 4) can’t say his abcs, count, and he is extremely rambunctious. I don’t blame him because my brother is 18 and had his very young and doesn’t even try, and his mother is a POS. She’s 21 and acts 13. I try when I’m with him to kind of teach him and be there for him but honestly it gets nowhere because his mom will come in and completely disregard everything I’ve done.

For example, he’s constantly SCREAMING. He is the loudest kid I’ve ever met. He wakes my daughter up every time she’s napping and I’ll ask him multiple times to use a quiet voice, and then his mom will be a bitch to me for it?

I don’t feel like constantly yelling and screaming is normal anyways during play?? But idk. I’ve never witnessed anything like it.

When he’s over he will just walk into the baby’s room without knocking or asking or go right by her door and yell. He doesn’t do anything but yell, and his mother again never corrects him and she’s loud as fuck too.

My niece asked him “what’s my name?” And he replied “dumb bitch!” And his mother found out and says “yup that’s my son”....... all proud?

He won’t sit at the dinner table and eat, ever. He just wants sweets and candy, and he gets it. His mother doesn’t give a shit. She will let him eat 4 brownies in a row.

I try talking to her and saying he needs to behave better and she uses the excuse that he’s only 3 and this is normal behavior for a 3 year old. I’m not sure I agree but my daughter is still very young so idk what to expect from a 3 almost 4 year old.

What’s normal and what’s not?