D.B.
You sound like a concerned sister, and you are to be commended for doing what you can (reading, singing the ABC song, etc.). I do want to caution you that it's not important to give a child academic skills at 3 - you should read for pleasure, put some music on or tune in to a kiddie TV show with sing-along fun, maybe go to the public library to take out a book (kids think this is amazing, a new book each week!). You can ask questions like "What does the cat say?" and "What is that dinosaur doing?" to see if he will converse with you when he's not stressed out and trying to get his own way. But make this fun - don't make it about learning to count to 50 if a child is this young. If you want to count to 3 while playing hopscotch or play an age-appropriate board game in which you roll the dice and recognize the "4" and then move 4 spaces, fine. But make it light, and fun.
I don't know if your brother has a speech or hearing problem or not, but there are early intervention services for kids who need it. The question is, will your father and stepmother check into anything? He may just be using what works - crying and tantrums. That won't last forever. This is beyond your control.
A child who isn't potty trained at 3 is less of a concern. Many kids with active and involved parents don't train until closer to 4, and that's just during the daytime. So I'd let that go for now. Eventually he will get tired of being wet, or they will get tired of paying for diapers, or he will not be able to go to pre-K or kindergarten if he can't use a toilet.
As you have noted, your brother has a parenting problem - he has parents who don't want to make the effort, who don't want to do the hard work of parenting. That means saying no and not letting the child run the show. Your brother is going to have problems in preschool and public school if he thinks crying is the answer to everything. So unfortunately, his parents are going to have to deal with this at some point. And of course he's going to have physical problems and dental problems if he just eats junk food.
What you CAN do, on your own, is write a letter to your brother's pediatrician. Maybe you can quietly find the name and address in the house - on an appointment card, on a list posted for the babysitter, on an enrollment form for a preschool, anywhere. Quietly write this down. Then when you get back to your house, write down your concerns with specific examples. What words CAN he say? Is he interested in reading? Does he sing along with you? What words won't he use? What does he eat? Do say to the doctor that you are a concerned half-sister who lives elsewhere but who visits occasionally, and you are very worried. Now, you have to understand that, legally, the doctor cannot respond to your letter nor can he/she talk to you if you call. But the doctor can (and will) use this information to guide the types of questions asked at the child's physicals and any sick visits, and to check for things like weight gain, developmental delays, dental issues and so on. Even asking about discipline and social skills is part of a regular physician's visit, so the pediatrician can put a little more emphasis on things as needed. A doctor might, for example, recommend some sort of hearing test or see if your brother can do certain tasks - often a doctor will just ask the parent, but in cases like this, the doctor may go a step further. It may be that this boy's parents need some parenting classes or some warning from a person in authority that some areas are being neglected.
If you do write this letter, my suggestion is that you not inform your father that you have done so - that doesn't mean to lie if you are asked outright, but just not to volunteer information. From what you've said here, I don't think there is a reason to involve Social Services (no matter how annoying and frustrating you find the situation). And you are only seeing his behavior on the weekends when you visit, not all the time. So perhaps he's not this way all the time.
