Celery is Evil?
From my SIL: NO ONE likes celery.
Tell your SIL she is incorrect. I love celery in soup, in salads, plain or with peanut butter and raisins or with blue cheese dip. Or any old way you want to serve it to me. I love it in tuna salad, Waldorf salad and potato salad. I love it in sage dressing at Thanksgiving.
If I think of any more, I'll chime back in again.
Celery is essential in mire poix, many soups, stews, chicken and dumplings and dressing as jmcarthur already mentioned. I gew my own this year for the first time and they really are beautiful (and tasty) plants.
Has your SIL had braised celery? Very good.
How does your SIL feel about celeriac? I adore it. But I know that wasn't your question...
ETA: Your SIL can use celery leaves to dry to make celery salt which. I use the leaves, too, usually to flavour soups/stews or as a main component of salad. So nice with goat or blue cheese, toasted nuts, etc.
*I* love celery! In my potato soup, in stuffing, in stir fry, raw with blue cheese dressing - lots of things I like to use celery for.
Heck, I need celery seed for potato salad too!
Raw celery by itself may not amount to much, but as a crudité for dipping, when braised, as one of the three elements in mirepoix and the Cajun holy trinity, and many other uses, it's one of the kitchen's more valuable players. (Others have already said much of this but it's worth repeating.)