I believe (at least from what we already saw in many houses) that normally the relatives know what's going on... And it's always the best thing to do (tell them) before they could know by others peoples
Even today i saw an old lady at Pavel&Alice that must be (i guess) mother of one of them
Was the guy that she appear for the first time (can't remember if it was her boyfriend from the time and breaking up with him after was the 'thing' that make her come to an apartment or if that time David was a 'friend' and she had broke up with boyfriend a few time before - according to groomy the end of the relation was what make it possible for us continues to watch her).
If you have access to archive, check page 77 (july 9) from Alex and Anna
Well indeed the only time she had 2 dicks at same time was with David and Alex and old apartment of A&A (And even Alex in that time was only blowjob). The foursome with Dean, Bree and Drew for her was just a threesome