Flyingkiwi,
Im not sure where you've got this idea about speaking about engine failures from. You dont get on a typical commercial flight to listen to a captain babbling about engine failures on take off....or aborted take off's...uncontained smoke or fire on start up etc
You'd frighten the life out of nearly every passenger on board. Perhaps whilst training for your PPL you've been in the habit of giving a brief to your instructor regarding emergencies. You are now however giving a brief to a
passenger and not an FI or examiner. As others have suggested, if you think about the brief you get on a typical airliner....how the seatbelt works, where the exits are, and where the life jacket is and how it works.....thats about it really. As for air sickness, I never mention the word. I do however tell them that if theyre too hot or too cold, or feel a bit stuffy then please say and i'll adjust the temperature. I then tell them theres a fresh air vent if they need it. They can read between the lines. Assuming they want to be a bit more than just a 'passenger' then brief the controls etc accordingly and tell them where to place hands and feet. Just use common sense without scaring them. If its a first flight for them, remember that they'll probably be excited and a little anxious so dont overload them with lots of info...as they wont listen! I do typical gift voucher trial lessons for people and you'd be amazed at how quickly it go's in one ear and out the other.