There should be a clause in the contract you're interested in adding a family member to that is titled "Assignment." That clause covers the transfer or assignment of the contract and is in the "Clauses" section of the ten year contract for the newer style ones; I'm not sure where it is for the older ones. In the newer ones it defines immediate family members as parents, children and grandchildren and makes transfers or assignments to them or heirs free of the transfer fee; since it's OP's brother, he wouldn't ordinarily be in the immediate family member definition, but could qualify under the "heir" portion if that's how things are set up. That's based on a contract I've got rather than the actual Mayan Palace one that this thread was based on, so it's important to read what it says in the actual contract you're dealing with. I did have one member services fellow tell me that the "heir" thing is only good after someone has passed and you would have to have proof of the inheritance, but I was only inquiring to find out what the bounds of my assignment clause really were as it doesn't seem to cover things like adopted children, etc. It is probably subject to interpretation and you could probably persuade them on the subject if you really try.
As far as who to contact, that is also covered in the contract; this time in the internal policy portion that is titled "Reservations and Customer Service." In the contract I'm looking at it states that
"[t]he only official channel for questions, reservations and all other communications with the Company regarding Member's Contract is the Company's Contact Center in Mexico." Then it provides the email
customerservice@vidanta.com or
reservations@vidanta.com and the phone numbers and hours of operation for the call center as well as the registered mail address Paseo de la Marina Sur #220, Marina Vallarta, Puerto Vallarta, Jalisco 48335 Mexico. I've upgrade a couple of times so have old copies of contracts and they all provide the same contact address although the name of the mailing address varies; the email addresses on an old Mayan Palace contract might be different, but the phone number is probably the same. Bottom line is read what the contract says about contacting them and follow the instructions there; the corporate headquarters for Vidanta is in PV/NV, which is where the contracts I have say to make contact with them for questions and all other communications.
We were told in January 2019 during our last visit that we had to call the Member Services at the resort we bought into. For example our contract starts with the letter "I" which means we bought in Riviera Maya. In order to add our children to our contract we had to get the paperwork from Riviera Maya. The contract automatically goes to your children however in Mexico if the kids names are not on the contract then the timeshare will be held up in Mexico's equivalent of Probate Court until the transfer is completed. We were told that takes about 2 years. We have no idea if the information given is valid as the salespeople change the rules with every update.
I think you're right to question the information you got from sales. Also, the letter "I" is the first letter for contracts sold recently for international guests and isn't specific to Riviera Maya. I believe the second letter is the one that designates where you purchased/upgraded; I think it is "N" for Nuevo Vallarta/Puerto Vallarta (PV doesn't really seem to do sales now) and "C" for Riviera Maya. I haven't figured out the rest of the lettering system before the numbers in the contract numbers, but that's the pattern I've noticed so far. Also, calling member services in Riviera Maya strikes me as being against the terms of the contract I had that was written in Riviera Maya, so I wouldn't do that. Check what your contract says, though, of course.
