My brother ended up got married 5 years ago. We are both disabled and were getting his retirement benefits from my father's record. He was always receiving SSI while I have always received SSDI. My brother got married February 2014 and his benefits stopped. My benefits continued as they have been, but my brother always received more of a benefit than I did I am guessing because he received SSI. Social Security now claims that they reduced the monthly benefit starting the month he was married and now they want me to pay back thousands of dollars. How can a sibling's marriage mess up the other sibling's benefit if they received the same benefit all along?
I can't be sure without more information, but from your description my best guess is that your brother's disabled adult child's (DAC) benefits weren't stopped timely when he married. If he didn't repay the resulting overpayment, then Social Security can pursue collection of the overpayment from anyone else who is receiving benefits on the same record. That is referred to as contingent liability (https://secure.ssa.gov/apps10/poms.nsf/lnx/0202205005).
The above explanation wouldn't apply if you're receiving Social Security disability (SSDI) benefits based on your own work record, but if you're receiving DAC on the same parent's record on which your brother was overpaid then Social Security could propose to collect your brother's overpayment by withholding your benefits. If that's the case, you may want to consider filing for a waiver of your liability for the overpayment (https://secure.ssa.gov/apps10/poms.nsf/lnx/0202201019).