This can often be a matter of perception.
Remember, no-one gets out of bed weighing 33 stone, it creeps up on the individual very slowly. If her parents simply think she is getting bigger, they will buy her bigger clothes, and simply ignore the fact that their daughter is dangerously obese.
From the girl's perspective, she may have low self-iamge issues, and feel unable to talk to her family about them, and before long, you have a classic denial situation, where no-one talks about anything concerning this poor young woman and her weight issues.
It is easy for us to step in at the stage, throw up her hands and say "How could she / they!" but life is rarely that simple.
What is needed is some practical advice for parents who may simply lack the skills to acknowledge and deal with such an issue.
Everyone involved deserves our sympathy, but being the 'fatist' society that we are, I imagine the Sunday red-tops are gearing their headings as I write, and such useful and helpful terms as 'gut bucket' will be shrieking at us across our Sunday breakfast tables.