I don’t think people should be calling her out for cancelling a trip to a country so far away. Yes it may seem bizarre to cancel so soon after she has just spent time over there but with something like a brain tumour, her mum may have very sadly just taken a turn for the worst.
She clearly loves her sister and I highly doubt that she would have taken the decision to cancel and break her sisters heart if she absolutely didn’t have to. Just my impression of her, they are all very family orientated which I do find a redeeming quality in Elle.