The color of water
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That's difficult to second guess motivations of of people, even characters that we get to know well. Judging from Ruth's indomitable spirit and sense of social justice, it is not surprising that her kids were involved in civil-rights. I think people passionate about such things make the cause part of their identity. Also you are right, fighting for these rights may well have filled a void from their past. I think though, it was mostly their mother who inspired them to fight for better and never give up.