The way I read the line it seems that LR fears that Anne will never get to meet a suitable man worthy of her estimation of Anne who might propose marriage, because of the circumstances Sir Walter has created with Anne always at home.

From Ch 2 "... Anne had been too little from home, too little seen. Her spirits were not high. A larger society would improve them. She (LR) wanted her to be more known..."

and just higher up from your own quote,

in Ch 4 " .... No second attachment, the only thoroughly natural, happy, and sufficient cure, at her time of life, had been possible to the nice tone of her mind, the fastidiousness of her taste, in the small limits of the society around them...."