I've been wondering how things are going and if you got them home with you yet. Sorry to hear you're having difficulty. I'm trying to remember what type of vehicle you have. Sometimes it's much easier with a van type rather than a PU or SUV until they're used to the "big jumps".
Whisper went through a spell when he was younger when he all of a sudden started to tremble when I attempted to put him in my Jeep. Even when Rainbow was already in. He would back up as far as he could on the lead. It broke my heart to think how terrified he was.
It took about a week of several lessons a day (3 or 4) using super good (hamberger) treats. I have no idea what triggered it all. But I think it was perhaps a bit of motion sickness associated with the vehicle + associating it with the Vet. The first time I literally picked the big guy up and pushed him in. (This was me - his Mom - not someone he didn't know or trust - like your Brother.) After he was in, I sat with him on the tail gate and reasured him. Calmed him. Talked to him and told him it was OK & what a G O O D = B O Y he was. Offered him the treat (which he didn't take at first) Then let him out & praised him again. Time after time until I felt he was ready for the "ride" we did this. The first ride was to the end of the lane and back. More treats. GOOD BOY!!!!! And so on.
Maybe you could put a really short lead on him (like 2 ft) as you would with a puppy to get him used to having it on. He could drag it around with him to get the feel and not be so frightened? Guess I'm rambling here but I really feel for you and the Puppers.
It's all going to work out alright. But in the meantime you must be extreemly frustrated. I know how much you're looking forward to their being home with you.
I wish I was there to actually help.