That still points to whatever the OP and mother digested (not on the aircraft), no?

How many people ingested the salad and the duck among other things on that flight? While 'specific' food poisoning cases can occur (maybe it came from the food preparer), the food is prepared in batches. If it was something from the food, you would expect (key word: expect) to see a much wider distribution. I'm not saying it's not plausible, but it's extremely unlikely and much more plausible that it came from another source.

When you look at cases that can be pointed back to an airline (or a cruise ship), many more than 2 fell ill (and they were certainly not from the same family / same experiences the days leading up to the flight). That's when the flight crew / the airline knows something is wrong as a "for sure".

As of now, it is what it is, inconclusive. If there was just one more case on the aircraft that didn't come from the OP's immediate family, I would immediately suspect UA. But I doubt UA will confirm if the OP asks, unless the OP is able to confirm others got sick on the flight as well. It would be really obvious.