There is a useful article in the Wikipedia concerning titles and Styles in the UK.

And an equally useful one concerning Forms of Address in the UK.

This explains why Sir Horace, the younger son of a baron, has a title but his wife does not. The widows of Anne's brothers do not have titles because their husbands were not the sons of the earl, but only the sons of the earl's younger brother.

As with any wiki articles, they are only as reliable as the last edit.