On the topic of nausea/vomiting:
Over the years, we’ve seen this happen a few times: after one family member vomited, one or two others also experienced nausea or even vomiting.
We were unable to identify a single underlying cause, but:

My observation and hypothesis is that the mere disgust one feels while cleaning, observing, airing out a room, etc., can trigger the same SBS (and one feels the need to get rid of this disgusting thing one is dealing with = nausea/vomiting).

Or that it happens on a purely instinctual level: the subconscious senses that another pack member is about to vomit. Archaically speaking, this is always due to the fact that a harmful morsel has been ingested. Since we are part of the same pack, there is naturally a risk that I, too, have eaten the same thing and now have a harmful morsel inside me—so for this reason, the same SBS is triggered as a precautionary measure.

=> So in the past, I always reacted to this by feeling nauseous. The last two times, I didn’t, because I
a) immediately open the window and dispose of the vomit as quickly as possible (to counter the disgust)
b) I actively remind myself that the vomiting isn’t caused by toxic food, but has an emotional cause, because an SBS was triggered by something I experienced. And that there is absolutely no danger to me regarding what I ate.

The issue of diarrhea can have a similar mechanism.