Question

Why is my brother wearing a diaper and using a pacifier even if he is almost 6 years old?

Answer 1

Your brother wearing a diaper and using a pacifier even if he is almost 6 years old is perfectly normal and it's likely because he loves to wear his diapers and loves to use the pacifier or even simply finds it comforting.

If your brother enjoys wearing the diapers and using a pacifier even at 6 years old, he should do so as it makes it happy.

There's plenty of other kids your brothers age and even older that like to use a pacifier and wear diapers. 

5 year old boy wearing diapers and sucking on pacifier pictures

And here's a link to a boy named Diaper Boy Tommy wearing diapers and using a pacifier and bottles.

8 year old Diaper Boy Tommy Wearing Diapers Pictures

And pictures of a boy around 7 years old wearing just plastic backed diapers.

7 year old boy wearing plastic diapers pictures

And some pictures of a boy named Jared wearing his Pampers Diapers. 

Diaper Boy Jared wearing Pampers plastic diapers pictures

Answer 2

Your brother could be wearing a diaper even if he's almost 6 years old due to incontinence or even for bed wetting.

Even kids who are autistic and those who are mildly autistic can take awhile longer to potty train and get out of diapers.

Is your 6 year old brother autistic or has autistic traits?

As for the pacifier it could be a coping mechanism, especially for anxiety or autism as I've known some kids with autism and those with social anxiety and other anxiety disorders that used pacifiers to help them cope even past age 10 years old.