(Snip)
] I don't quite recall but I don't think Darcy is thinking of marrying his sister to Mr. Bingley. It is only Caroline Bingley who mentions it in a letter to Jane and makes it seem that way.

Actually, Darcy does have Bingley in mind for Georgianna. JA explains this towards the end of a rather hard-to-puzzle-out paragraph in Ch 45. I'll quote it here, with clues as to each person referred to. The paragraph comes just after Miss Bingley tries to upset Elizabeth by referring obliquely to Wickham, and E is able to respond "in a tolerably disengaged tone." ...

...Not a syllable had ever reached her [Miss Bingley] of Miss Darcy's meditated elopement. To no creature had it been revealed, where secrecy was possible, except to Elizabeth; and from all Bingley's connections [relatives] her brother [Darcy] was particularly anxious to conceal it from that very wish which Elizabeth had long ago attributed to him [Darcy], of their [Bingley's relatives] becoming hereafter her [Georgianna's] own. He had certainly formed such a plan, and without meaning that it [his plan] should affect his [Darcy's] endeavour to separate him [Bingley] from Miss Bennet, it is probable that it [Darcy's plan] might add something to his [Darcy's] lively concern for the welfare of his friend [Bingley].