HI to whom ever this may concern.
I emailed directly to the Riu Palace Mexico AND the Riu Palace Riviera Maya. They have assured me that
A: there is no additional charges for someone attending a wedding at the RPM and yet staying at the RPRM
B: the only additional costs would be if the wedding couple is throwing a meal in a private restaurant.
If anyone needs the information I can forward you the email and or the email address of the person/s I spoke with regarding this matter.
Originally they told my neice she would have to pay extra BUT when I emailed in to say we were staying AT the RPRM and attending a wedding at the RPM,,,,it was confirmed not once but three times that there is no additional charges for attending the wedding. So you can stay at a higher RIU than where the wedding is taking place and not pay extra.
To date the RPM and the RPRM are sister hotels and of equal value so there is no charge to go back and forth between the two other than to go to the private restaurants.