What is the ruling on common sayings such as "sweeping children brings guests" or "an itching lip means guests are coming" and the like, and what is the ruling on joking about them?

It is not permissible to believe in the causality of anything unless its causality has been established by revelation (Shar‘an) or by sensory perception (Hissan). This includes the notion that children rubbing their lips is a sign of guests arriving, as there is no evidence for this. It is also not appropriate to joke with such phrases, so that the general public does not believe them to have a religious basis.