To me, it does seem that Zenaida is referring to Ella. Perhaps their friendship had weakened over the years, or maybe Ella had been getting on Zenaida's nerves in recent months.
While it is true that Ella was busy with her good works, she also spent time with various family members, and since she was close to Felix she might well have considered Irina something of a surrogate daughter, or perhaps a social equal.
I believe something is mentioned within the OTMA correspondence here on this website about their Aunt Ella sitting in a chair, fast asleep and snoring. (But now that I've mentioned it, I'll need to check for it . . . oh dear!

) Again, my point in mentioning this is that while we see Ella as something of an icon, Ella would have been seen by various family members as who she was to
them-- i.e., an older aunt, a shirttail relation who showed up at inopportune times, etc. I think it's also entirely possible that Zenaida rather resented Felix's treating Ella as a sort of surrogate mother, and therefore vented this resentment to her only surviving son via the letter. Also, since Ella was tremendously respected for having taken the veil after her husband's assassination and devoting herself to good works, perhaps at that point Zenaida was expressing some peevish envy by using the "celebrated sister" remark.
Just my "take," however . . . will be interested in other theories!