Use Form 2120 to:
Identify each other eligible person (see below) who paid over 10% of the support of your qualifying relative whom you are claiming as a dependent, and
Indicate that you have a signed statement from each other eligible person waiving his or her right to claim that person as a dependent.
An eligible person is someone who could have claimed another person as a dependent except that he or she did not pay over half of that persons support.
If there are more than four other eligible persons, attach a statement to your return with the required information.
For more information see IRS Form 2120