“…and Niobe would have been the most fortunate of mothers, had she not seemed so to herself.”
…et felicissima matrum
dicta foret Niobe, si non sibi visa fuisset.
Note: Niobe had fourteen children, and on this account, boasted of her superiority to Leto, who had only two (Apollo and Artemis).
The phrasing here may remind frequent readers of this site of Tacitus’ comments on Galba: capax imperii nisi imperasset. http://sententiaeantiquae.com/2014/07/17/tactitus-histories-1-49/