Sometimes I add non-musical family in order to link two musical people together.
Is there a way to mark the non-musical entry so that other sites, such as BBC, don’t add the non-musical person?
Not really. But I wouldn’t worry about it. The BBC will have plenty of artists it’ll never use, this is just one more.
I don’t think this is something we (as “upstream”) should worry about. “Downstream” users of the data can do their own heuristics for who to show (or not show). E.g., they can decide to not display artists that only have family relationships and are not in any artist credits. (And similar for photographers, graphic designers, etc.)
Each consumer of our data uses the data for something different. An example @Rob likes to use is that it is possible to use our data to solve the command “Hi [virtual assistant of choice], play a random album by a son of Bob Marley.” This could well be extended to “grandson”, “niece”, “cousin”, or even just “family member”. If a downstream consumer excluded any family-only artists, they could have trouble solving queries like this.