couldnt you just catnap the kittens without someone seeing you,then maybe neighbours might just say someone picked them up, worth a try
as no doubt she'll just let the brother and sister mate and the cats will be deformed then god knows what she'll do

have you not seen the price of kittens these days, some are going for £100, one woman was asking £60 my daughter phoned up and asked if she would take £40 she said no i want £60 as the pet shops are asking £120