What papers does the irs need to prove that I provided for my niece and nephew the whole year and that they are related and live ith me

If they're under the age of 19 (or 24, if a full time student) you don't need to provide the amount of support, just that you were the custodial parent and that they lived with you more than 1/2 of the year.  That can be done with school records, medical records, affidavits from neighbors knowledgeable with the situation, etc.

None of this needs to be sent in with the return.

The letter should specify the documentation needed or allowed but simply put ...basically anything that supports your right to claim the children .... https://www.irs.gov/taxtopics/tc654.html

http://budgeting.thenest.com/documents-needed-prove-irs-child-yours-21745.html