While Col. Fitzwilliam is not himself titled, he is the younger son of an Earl, so of high enough social standing to be 'equal' to Anne in marriage. Her sickly condition, apparent lack of 'accomplishments' or beauty - and perhaps her overbearing mother - make it unlikely for her to attract a man on her own merits, and an equally wealthy man of her class would perhaps be more able to marry where he likes and not be attracted by her wealth. But her wealth would certainly be a drawcard for a man such as Col. Fitzwilliam, plus as you say, Lady C was not opposed to marrying within the extended family. I wonder what was the marital status of Col. F's older brother, the future or present Earl - perhaps Lady C would have started planning for his union with her daughter !! :) .