IMO, LR couldn't care less whether CW marries someone good enough for him, it's Anne being appreciated that she cares about! ;-) I suspect that she probably thinks CW couldn't *really* have appreciated Anne in the first place, if he could now fall for someone like Louisa. She probably thinks that CW has been "captivated by youth and beauty, and that appearance of [insert virtue here] which youth and beauty generally give", and that possibly Anne's beauty may have had too much influence on his love the first time.

As for Louisa being good enough to become Anne's sister, again I think that JA believed that you could (and should) choose your spouse, but once married you accepted the in-laws you got (and after all, you could do far, far worse than Louisa!). IMO, anybody would have much higher standards in choosing a spouse (either for themselves for a beloved "child") than in choosing the in-law(s) who come with that spouse.