If the information came from your son, and even though it sounds like he may not really want to talk about it... Could you bring up the subject with him, and see if he'd be comfortable passing along to your daughter that you're in the lifestyle as well?
I think this is something that needs to be discussed, especially seeing how close you live to each other. My parents live an hour away, and while I don't think that they're in the lifestyle... They don't know that I'm thinking about jumping in. My mom will probably hear about it eventually... She and I have had an odd relationship. As I have gotten older, things have opened up a lot. So much that I was able to talk to her about the problems, sexual and otherwise, that I had with the ex-husband.
It's a tough call, but given the proximity, I'd rather be safe than sorry. If I was 20 minutes from my parents, I know I'd want to know if they were in the lifestyle as well. In fact, that's part of why I'll eventually talk to Mom about it... because, sometimes, you never know...