Conversations with vulgar words

I have some relatives and we talk nearly every single day. And they keep cursing while i keep saying not to do it. (They already know it is forbidden) I keep telling them to shut their mouth, but now that i think about it, it's kinda harsh, would like to know if it’s REALLY okay to say that. Anyways, can i still talk to them even if they continue swearing and am i obliged to keep telling them to not do it every single time they do?