Which preposition fits the the sentence I was visiting my sister in/at the hospital?
They are both pretty acceptable. At the hospital suggests that you were meeting within the entire hospital grounds, where as in the hospital referrs to inside the actual building itself.
Thanks, Zantor.
Hi mishy
By the way, in hospital can also be used without preposition when we say generally and don’t mean any particular ‘building’ 
She has been in hospital for a week.
Yes, that’s one of the differences between British English and American English.
In the US it would be “She has been in the hospital for a week.” 
Hi,
You wrote:
They are both pretty acceptable. At the hospital suggests that you were meeting within the entire hospital grounds, whereas in the hospital referrs to inside the actual building itself.
What is the differenc in meaning btw inside and within?
In my dictionary, within = inside :?
Quoc
I meant the difference between the actual buildings and the entire hospital grounds.
Hi,
Yes, in a small dictionary they would probably be regarded as synonyms.
Alan
You must have meant 'without the article’, Tamara. 