The following quote gives some insight into Rosa becoming an activist and returning to South Africa regardless of her parents' position:
Nothing can be avoided. Ronald Ferguson, 46, ex-miner, died on the park bench while I was busy minding my own business. No one can defect.
I don't know about the ideology:
It's about suffering.
How to end suffering.
And it ends in suffering.
As the quote above indicates, Rosa decides to go back to South Africa to be involved with politics. The quote also suggests she does not follow her parents' position of orthodox Communism. She returns to become an activist not because of her parents or their politics but because, as the quote states, "no one can defect." She says she doesn't know "the ideology" beyond that it is about suffering and ending suffering. She goes back to the activist work because she simply has experienced too much to shut her eyes to it, and she can't divorce herself from her need to be committed to changing the system in South Africa.
At this point, Rosa has experienced not only the beauty but the narrowness of the bourgeois lifestyle and been challenged by Baasie to do more than merely trade on her father's fame. She goes back because she cares about what happens to people under apartheid.
Her time in France is a period of testing. As a child and teenager in her parents' household, she had no choice but to center her life around political struggle. France offers her a romantic, beautiful alternative of escape into bourgeois life. Though she has another option, she chooses the harder path of activism because it is central to who she is.