Peeing from overexcitement or as a display of submission is fairly common in young dogs. The peeing will go away if you address the underlying reason for it. Some dogs automatically detect certain people as strong leaders. They will urinate in front of them as a show of submission.
Some dogs urinate as a result of overexcitement. For that, you need to train your dog in how to properly greet visitors and not allow excitement to escalate. Any excited behavior - barking, running around, and jumping on people - should not be rewarded with petting or talking. That's rewarding the behavior you don't want. Ginger is spot on in her advice for visitors to initially ignore the dog: no touch, no talking, no eye contact until the dog is calm. A dog who approaches a visitor for a sniff and a pat and then either walks away or lies down is the behavior you're looking for.
As your dog's pack leader, set up visitor exercises. Put your dog on the leash and have someone come to the door. Go to the door and have your dog sit. Do not open the door until he is sitting calmly. Invite the person inside. If the dog stands up, tries to jump, or barks, pop and immediately loosen the leash.as you sternly say, "No." Return the dog to a sit and continue correcting the behavior you don't want until the dog is calm. It's even better if he voluntarily lies down. Your visitors should simply stand and wait for however long it takes for the dog to calm down.
At the time, it seems like it takes forever, but in reality it's usually only a few minutes for the dog to give up and calm down. You want him interested, but calm.
Excitement like you describe isn't always a good thing in dog behavior. It doesn't always mean the dog is "happy" to see you (or any other visitor). Excitement can also be a sign of fear of new people. Without seeing your dog myself, I can only give you examples of what it may mean. Aside from peeing around these two people, what is his excitement level in the public places you take him? Does he run around the park like a mad thing or does he reliably come back to you when you call? Does he "charge" other dogs in an attempt to play or does he hang back waiting to be approached? Is his only exercise an energy burst at the dog park or do you walk him regularly? There are a whole lot of clues to be had from your dog's general lifestyle and behavior that might help pinpoint his problem with the two visitors.
If you haven't begun dog school yet, it's definitely time to go. Dogs are little sponges in their first year of life where training is concerned, so don't miss this opportunity to properly socialize and train him. Training is actually about teaching the owners to effectively communicate with their dogs, and to establish you as the pack leader who is to be respected. You can't earn that respect unless you learn some basic communication skills. It's really easy and a whole lot of fun too. :-)
I know this sounds strange, but you could try this. When either your sister or your neighbour meet your dog, get them to ignore him when they approach and not to look directly at him or make eye contact. And only if/when he is calm, and has relaxed some, then they can gently stroke or pat his head as a gesture of affection. See what happens.
From a human point of view that seems vaguely mean, but from a dog's viewpoint, what it might do is help to take away the excitement, balance the situation and take away any stress. Because even though your dog is pleased to see them his excitement is causing him some stress.