One of the ways I have located relatives along with associated family members was through the DC Tax and Revenue website - Washington DC Online Public Records. It’s not so easy to find so once you do you should bookmark it. http://otr.cfo.dc.gov/otr/frames.asp?doc=https://gov.propertyinfo.com/DC-Washington/

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Click on Land Records
Search by Grantor/Grantee

This database goes back to 1921. If they owned property in DC you may be able to find them and/or other relatives on the property documents. You can also use the square and lot numbers from those documents to locate the address, because the address is not always there. To do that you need to go to Real Property Service Center - http://otr.cfo.dc.gov/otr/cwp/view,a,1330,q,594366,otrNav,%7C33244%7C.asp

Real Property Tax Database Search and enter the square and lot number for the address and current owner.

If your family member is not living you can some times find the married names of female relatives. Its important to look at all related documents because usually when a name is removed and the rest of the family is still there it is an indication that the person is deceased. Property owners usually have a number of transactions over the years.

Good Luck!