Does the book tell what happened to Jackie?

Yes, I agree that Margaret would have asked Henry to (anonymously perhaps) make a settlement on Jackie that would allow her to live comfortably for the rest of her life. Or perhaps Margaret herself gave Jackie something. My question is, did Jackie ever find out that Helen's child was Leonard's? It's not answered in the book, but I've always wondered.

re: "....the way she takes a piece and seductively puts it in her mouth...)."

Actually she's trying to be 'a lady', eating daintily, or she thinks.

I have just finished reading the book, and my take on Jacky is that Forster's silence about her indicates that after Leonard's death she fell back into the squallor and prostitution from which Leonard had, by marrying her, rescued her. Helen and Margaret were interested in helping Leonard and Leonard only, because they saw in him an intelligent young man who was held back by the social stratification of the society in which he lived. Jacky, on the other hand, they saw as a stupid and boorish woman and she evoked no positive sentiments from them. Leonard had been their pet project in a way, especially Helen's as she became more and more caught up in Leonard's fate and enraged by the stoic indifference of the wealthy, such as Mr. Wilcox, towards the lives of the poor. But by the end of the book, the sisters had both become disillusioned with their cause, I think because they realized that they were in truth, unable to do anything for the poor, no matter how hard they tried, and Leonard's death only solidified in a tragic way this realization. They were past philanthropy by the time Leonard died, having seen how utterly useless their good intentions had been and I doubt they even thought of Jacky following her husband's death, since she had never been a part of their cause, just a consequence, having been married to Leonard.

Leonard's death was very tragic, but I think that Jacky was the most tragic and pathetic character of the book, since she was nothing more than a side-note to everyone around her, a persona non grata, for even Leonard did not love her and stayed with her only because he pitied her and honorably stuck to his promise of marrying her. Sad as it may be, I think that her character shows that there is something to Henry's rather callous statement that, "The poor are poor, and one's sorry for them, but there it is", which the sisters probably never fully accepted, but I got the feeling that they had stopped worrying about and trying to change, as they had their own problems to worry about by the end.