There are two things being confused in this thread. 1 is data retention and 2 communication.
Data can be retained under a number of categories. One of which is "consent" but the category most applicable to RMWeb (in my opinion) is "legitimate interest". I am a member of RMWeb and would expect as part of that membership for some necessary data to be held. RMWeb can legitimately communicate with me in relation to my membership of RMWeb. The other common category will be "Contract" where if I buy somthing I would expect the person selling to me to hold my data in relation to that contract. In neither of "Legitimate Interest" or "Contract" is my positive consent required.
The second issue is communication, which for unsolicited marketing emails requires for the person/company to communicate in that way, with me, to hold my data with my positively given consent because holding data otherwise than "Consent" does not entitle the holder of my data to communicate other than in the restricted manner implied by the data category. Unsolicited marketing only falls under the "Consent" category.
BRM may or may not be a "sister" organization to RMWeb but they do not hold my data with my consent as I have not given my consent and they cannot assume that data held by RMWeb under "Legitimate Interest" can be used by them to send unsolicited marketing emails.
Indeed such practice is unlawful within the GDP regulations.