You might want to post a little more information so folks can provide better help.
Do you know the state, county, city where these folks lived?Were they also African-American? (If you're searching records, that definition is probably more recent than the relationship - you'll have to look for Black or Colored in records)
Any idea of their given names?
Perhaps Thomas named some of his children after his foster family.I'm assuming foster, Do you know if Thomas was adopted?
Since Thomas is your grandfather, maybe some aunts or uncles can provide you some more information.The first rule of genealogy is: "Talk to the living."