I agree with welbeck, so I'd be inclined to ignore any difficult behaviour - as far as possible. If she's sulky and refuses to sit at the table, I'd find that quite easy to ignore. It may be that she really needs to be there but just finds it impossible to join in. Show her understanding and affection rather than any nagging to conform.
Words you don’t often see now.
Venezuela earthquakes, is £2million all the UK can afford to send for the relief effort?
