Please tell me Raff will catch on now... after all, Tish even said that she's getting her supplies from tricksters...
I'm pretty sure at this point Tish and Leo know all about Ra, and are letting her prey on the locals as long as she doesn't do permanent harm; after all, vampires have to eat, too, and Liara looked all right last we saw her...
I'm not so sure that they know much about Ra. Ra has likely only encountered these guys twice and she's not the only one who needs sleeping dust. As far as they know, she's just a regularly weird customer.