I asked the following question to Josephine Tewson:

"I have heard so much conflicting info about this and would like to
know for once and for all-are you a cousin of the late John Inman?"

"No, John and I are in no way related. We once did a show together
called "Odd Man Out" and played step brother and sister and perhaps
that was where it started."-Josephine Tewson ("Keeping Up
Appearances")