Have you ever sat down with him -- not when he is in the throes of screaming because he's scared but when thing are calm -- and actually asked him what it is he fears? I'm betting you have already and he couldn't tell you. That's frustrating for us adults but very, very typical for a young child. He doesn't know what the problem is; he only knows the emotion he feels.
But with some good questions from you, you both might be able to put some names to his fears. Also look at your home from his perspective, not your own adult perspective.
Did something happen that he hasn't told you about? It might be as simple as bumping his leg in the dark when he went to the bathroom in the nighttime, and now he is irrationally scared of the dark bathroom. You need to draw out from him whether something like that went on.
Is it dark upstairs? Does he have to turn on a light to go into the bathroom? Then leave a light on in there all the time. Cost pennies but could help hugely.
What's the art on your walls? Pictures of relatives? Other stuff? A young kid's imagination is very powerful. A picture of the beach that you love could contain a shadow that to him is a monster. Your adult mind won't get it, but please take his impressions seriously and remove anything that HE finds scary no matter what you think. Don't try to explain things like "The eyes of the person in that picture really are not following you! It doesn't happen. Look, see, they don't really move." Instead, just listen and say "I can see how that would feel scary" and remove the picture or other object that he finds frightening.
Walk through every inch of your home with him - not to explain away things that scare him but to listen to him and immediately change things that scare him. LIke I said -- at first he may say "I don't know what's scary, I'm just scared" so don't press him too hard, but do give him some power. He feels powerless. So leave lights on; go with him when he wants you to (it does not make him weak if you go with him -- he needs you, so be there). Walk through every room and ask him "What can we do to make this room friendly and not scary?" If he can't answer, come up with things yourself.
Consider getting an empty, clean spray bottle to fill with water and let him decorate with stickers -- anti-monster spray he can use to spray in the air before he enters a room. Work with him on creating his own "spell" he has permission to say or even yell when scared: "Get out of here, monsters! This is MY house!" or something like that. These things may seem silly but to him they could give him a feeling of control.
Do not scold him. Please do not continue to make him stand on the stairs "until he's ready to go up" to the point he's screaming -- why do that? It only shows him you don't believe he actually feels real fear, when it's totally real to him. Making him stand there is punishing him for an emotion, and doing that teaches him that he should not reveal emotions to mom. You don't want him to learn that lesson, do you?
I would not send his sister to be his keeper. It sets up a relationship where he feels belittled -- his younger sibling has to look after him when what he really wants is mom. His sister also will start to resent this. Keep her out of it. She either could end up teasing him or adopting his fears herself, later.
You don't say if this fear is limited to going places in your home or if he also is fearful at school or in other places. Does he have fears of places in his school? Does he get terrified in public places, at church, etc.? If so -- yes, he may need professional help.
One other thing -- your family may forbid scary TV or video games, but he's in school; other kids are telling him scary stuff and you cannot ever control that. I would talk to him about whether other kids are saying things that are making him scared at home. You may need to remind him that it is not "tattling" if he tells you what he's hearing at school -- he may clam up otherwise, fearing that he's ratting on friends if he tells you that "Bobby says there is a killer in the shower at his house that jumps out at him and I'm scared there's one here too" or kid things like that. He will be influenced (and scared) by things you cannot control, now that he is school-aged, so be aware that having a peaceful and violence-free home life is no longer going to shield him entirely.