If i come with my 5years old brother, do i have to buy a ticket for him???

You can try & sneak him in, if the staff on deck wants to maybe they ignore him. As far as actual rules yes it's required you pay.

Yes and no....depends on the size of your 5 year old

Not if your sneaky. I take my 5 year old all the time and dont buy her a ticket they dont really ask.

Yeah you do. Unless they're doing the summer special. In that case he's free