My youngest went through this phase (he was 2). Many of my family and in laws are like yours, they dont care what they say in front of kids. I explained that this was a word for mummies and daddies not children and one day when he was a grown up he could use it - until then it was not allowed. He hasnt sworn since and since i didnt tell him NO or get upset he never said it again, however i heard him tell off another child for using it- he repeated what i had told him, the poor child looked shocked!!
This is exactly what we have said to DD and it worked. My family don't say **** etc but bloody, sh*t and crap come out a fair bit in normal conversation with my dad and my older sister. Dad tries to stop himself but he forgets, and my sister only visits from the NT once (sometimes twice) a year so it's hard for her to stop herself for a couple of weeks.
Most recently she called the onion in her spag bol "this crap" when telling her dad she didn't like it. He just told her that is a grown-up word and she should just say 'I don't like this'. That worked.