Swearing and a 3 yr old

Both myself and the parent of a 3 yr old I care for have a problem with her swearing. The 3 yr old has excellent language skills and can be very eloquent. Unfortunately the child's teenage sibling thought is was funny to teach her to say lots of swear words. We have tried all the methods we can think of to try and stop the behaviour (looked at copious childcare books). We have tried to ignore it with the hope that it would peter out but this hasn't happened. Her parent's assure me that they don't use bad language at home and I most definitely don't.
The older children I care for now ignore it (they've given up saying '......said a nought word', and thereby drawing attention to it), however I also care for 2 younger children, the older of which is starting to repeat some of the words, in fact yesterday they were playing in a tent in the garden and I overheard the 3 yr old telling the 2 yr old to say 'f*** off' and 'bu**er off' and I rapidly called them out to play with water.
Does any one out there have any suggestions that may help?