If you feel the need to make jokes about my mum, then by all means go ahead, she's very easy going, as am I. If people were tweeting the family directly, then I'd feel differently, but as it is, it's some jokes that nobody involved will ever see, so I really don't see there's an issue.
Why do people get upset on someone elses behalf ?. so long as you dont seek out friends,family etc or deliberately want to upset others then its OK , IMO. And as for Heather Mills mentioned earlier, the only good thing about her was that she was the source of one of my more favorite silly jokes "what has 3 legs and lives on a farm ?" - - - its ruined now they split up though.
