I don't know. It's hard to defend anyone who knowingly engaged in discriminatory practices (I'm taking the OPs word for Mrs. Sterling's involvement in housing discrimination)... but, we don't really know the specifics and extent of her involvement.
While I've never been party to a race discrimination scheme, there are things I've done in my life that I am not proud of that have belied my values (revealed my weakness perhaps, but belied my values). I know far too little about her to condemn her, or to condemn Doc for supporting her. As much as I could surmise negative things about her, I could also surmise that she got herself into more than she bargained for with Sterling. A decision made initially as a young woman -- in an era where men dominated women with immunity -- perhaps she was victim to his power. This doesn't excuse her, especially in later years, but does present at least some cause for compassion -- which is all Doc is showing. He's not nominating her for sainthood.