There is support amounts paid by you, paid by your potenetial dependent and paid by outside sources. If your parents are receiving SSI, this is money they are contributing to their own support. If a sibling is helping out, this support is given by them and does not count as your or your parents' support amounts.
If you are contributing between 10-50%, you are not paying over half. In the case where more than one person contributes, there is Form 2120.
https://www.irs.gov/forms-pubs/about-form-2120
