Hi mariababy
Your niece may be hit with a penalty charge for not reporting when she went into hospital, and hit with an over payment.
The first day of going into hospital is not counted, nor is the day that you are discharged.
If you go into hospital for over 28 days then you should notify DWP.
We would advise your niece to seek advice from her local Welfare Rights Organisation;
advicelocal.uk Gary