Magical Pests, cursed items, booby traps - Grimauld Place has a variety of dangers for the unwary. Some of which are subtle enough to avoid detection. For a family of Dark Purebloods, there are very few reasons why those of impure blood (muggleborn and half-bloods) would be "allowed" to stay - and Kreacher, a loyal elf, is more than able to follow such ancient traditions.
And so it is that, in the summer before fifth year, any females of impure blood -from the intelligent Hermione Granger, to the spunky Nymphadora Tonks, and potentially even a withdrawn Fem!Harry- fall afoul of the House of Black's subtle means of training a sex slave.
...Basically, Kreacher starts obeying old family traditions - which Sirius is unaware of and thus unable to stop. This includes things like enchanted headboards that cause the person sleeping on the bed to be trained in their dreams, potions slipped into their soaps, enchanted/cursed jewelry, etc.
Maybe even have Sirius fuck them all, but he thinks they're all just naturally submissive sluts that don't mind being cheated on or being the side ho.
