The entire paragraph:

Elinor's curiosity to see Mrs. Ferrars was satisfied. She had found in her everything that could tend to make a farther connection between the families undesirable.

Elinor had seen enough to realize that she didn't want Mrs. Ferrars as a mother-in-law.

She had seen enough of her pride, her meanness, and her determined prejudice against herself, to comprehend all the difficulties that must have perplexed the engagement, and retarded the marriage of Edward and herself, had he been otherwise free;

She realizes that if Edward were free and were to propose to her, Mrs. Ferrars would have made their lives very difficult.

-- and she had seen almost enough to be thankful for her own sake, that one greater obstacle preserved her from suffering under any other of Mrs. Ferrars's creation, preserved her from all dependence upon her caprice, or any solicitude for her good opinion.

She saw enough that she was glad, for her own sake, that an obstacle already existed in Edward's engagement to Lucy that would prevent Elinor from having to put up with Mrs. Ferrars.

Or at least, if she did not bring herself quite to rejoice in Edward's being fettered to Lucy, she determined, that had Lucy been more amiable, she ought to have rejoiced.

If she couldn't bring herself to be happy that Edward was engaged to Lucy she at least conceded to the fact that she would be happy that this obtacle existed if she felt Lucy deserved Edward.