In fourteen states includes Utah, why is illegal for man to marry his window sister?

Either because windows can't have sisters, or because you cannot be
legally married to inanimate objects, or because the man is dead.

I think that the original question should read 'In fourteen states,
including Utah, why is it illegal for a man to marry his widow's
sister?'

If this is correct, the following website may assist as it states the
Christian Church teaching that marriage is 'so close a bond
that it unites not only the married couple but their families. 
A man becomes the brother of his wife?s sister, the son of his wife?s parents. 
'  Therefore if a man were to marry his widow's sister, he would enter
into an incestuous relationship.

http://www.katapi.org.uk/ChristianFaith/LXVIII.htm 

If this ansers your question, give your suggested price to charity!

If it is supposed to be "his widow's sister," then the man is dead.
Otherwise he would not have a widow. Dead men are usually ineligible
to marry anyone.

I believe you're trying to say: "Why is it illegal for a man to marry
his widow's sister? -
If his wife is a widow - he's dead - so it's an impossibility.
Legality only deals in possibilities.

The legality of marriage with the deceased wife's sister was a big
deal in Victorian Britain.

Sarah Brown, University of Cambridge. "Deceased Wife's Sister Act."
The Literary Encyclopedia. 21 Jun. 2004. The Literary Dictionary
Company. 26 October 2005.
<http://www.litencyc.com/php/stopics.php?rec=true&UID=1430>