Any good transmission tech with the right equipment can get pretty close, if not exact, about diagnosis without even removing the transmission from the van. So no - diagnosis does not require disassembly. So if you’re worried about it, take it to your step brother - if you trust him, and if he’s a good transmission tech.
Beware of what passes for a transmission tech. I called a local “transmission” shop in my town once. I asked them about their experience with a very common issue in a transmission that I have. They professed to know nothing about it. It seems that their “specialty” is really just limited to fluid servicing and removal & replacement. Lots of places, including most general mechanics, are like that. They don’t bother. Anything that “acts” like a transmission problem is reason to throw the baby out with the bathwater.