Sentence:  You tell me you are secure of having either the Aunt or the Niece, and that you might have married the Aunt before this, whose <WSD>Jointure</WSD> you say is immense, but that you prefer the Niece on account of her ready Money. 
Meaning of the word marked with <WSD> tags:  An estate settled on a wife, which she is to enjoy after her husband's death, for her own life at least, in satisfaction of dower.