mr blobby
She may have been selected at random to make sure that she is receiving the right benefits or it is possible that she has been reported to the Fraud Line or that an anomaly with her Pension Credit claim has been flagged, this would be most likely to do with any income your household has.
Yes, you can attend with her.
Try contacting the JC+ advisor, they may be able to provide more information about what the interview is about.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
