Tillo was 'only' 7 kilo when he was 5 months. He's about 10.5 kg now. He would probably fit your new harness on the smallest size, so there is hope 
It is the owner who needs to make the adjustments necessary to obtain a desired behavior. Sometimes it is hard to figure out the "work around" and, it is incredibly easy for an owner to unintentionally reward an unwanted behavior. You can't be "sloppy" with a basenji - they train us well!
I adopted my b-boy about 3 year ago. He wants to decide which direction we walk. Sometimes, that's okay with me, other times it is not. He puts on the brakes and refuses to budge. At first I dragged him a bit. Not good. Treats don't work. Then, I tried giving a quick jerk/release. Not good. (If you try this, please be careful not to cause damage to your dog's neck!) What works for me is I just wait for him to realize I'm not going to give in (this takes A LOT of patience). Sometimes, while I'm waiting for him, I talk to him about why I want to go in a specific direction and tell him what's in it for him... people walking by think I am absolutely bonkers trying to verbally reason with a dog, but it works for us. Sometimes, he completely refuses to budge, so I pick him up and carry him for a bit. Usually, when put him on his feet, he will walk on. I noticed that if I walk to get behind him, he takes a step forward and once he makes that first step, he's more likely to move forward, When he does, I praise him like crazy. Sometimes, especially in dim light, he will stop, stare, hackle and refuse to move forward - at those times, we simply turn around and walk in the other direction because I trust that he sees or smells something that he finds dangerous.
One thing I consistently do that might help Woody is I always feed my b when we get home after we walk. That way, he has something to look forward to (i.e., going home is good!) I'm sorry Woody is taking so long to warm up to your brother. Maybe your brother needs to work with Woody on some very simple command like sit, or look at me, giving her high value treats or her dinner when she obeys. That way, she may begin to see him as more of a leader and provider of treats than someone who scares her. It needs to be brief, daily consistent to sink in. Patience. You might have to let your brother be the sole source of all good things (like treats and food) and only if she makes a step toward him. A trainer once advised my neighbor to smear peanut butter on her husband's arm in an attempt to get their new rescue to like her husband better. It didn't work.
I'm so glad to hear you see a difference for the good and that you are learning as much as she is. Yay! Please keep us posted on what works. Maybe you can help me with my stubborn b.
