Research Undertaken So Far: Googling peerage courtesy titles, duke's heir's wife, and duke's heir's widow have all been fruitless. Also, I've consulted a website that I've found incredibly helpful in the past: Peerage Basics to no avail and as for the first part of this question, I've looked at the 1662 printing of the Book of Common Prayer.
As far as I can tell in the "Table of Kindred and Affinity" in the BCP, it would have been possible for step-siblings to wed in the period that my novel is set. Is this true or do the terms "sister" and "brother" refer to step-siblings as well as half-blood and full-blood siblings?
As for the second question...my understanding of the peerage leads me to construct the following scenario; if it's wrong, please feel free to correct me.
The Duke of Cheeseparing has several children; the eldest of his sons would be the heir to the title and would thus be styled Marquess of Tinwhistle. Tinwhistle gets marred and his wife styles herself Marchioness of Tinwhistle, courtesy of her father-in-law. Lord Tinwhistle and his wife have no children and he predeceases his father, in which case the next eldest son (who up until this point has been known simply as Lord Alexander Harping) steps up and becomes the heir presumptive and is then styled Marquess of Tinwhistle himself. What title, if any, would the widow of the previous Lord Tinwhistle be allowed to use? Since the marquessate in question is basically on loan by courtesy from the Duke of Cheeseparing, I can't think that she'd be allowed to call herself Dowager Marchioness of Tinwhistle, but I also can't think what else she would be called by society. Lady Harping? Lady James Harping (as if her late husband had been a younger son of a duke)?
Any and all help will be greatly appreciated!
ETA: I've got enough information about the second questsion...don't need any more suggestions on that, thanks.