Can you join the tour in Sicily. I would be coming from Ireland and my sister from Australia.
What size are these tours
You need to sign up before you go... you do not just 'drop in'
https://www.ricksteves.com/tours/italy/sicily
"A small, friendly group of 24–28 people — half the size of most tour groups"
Any tour that is not wait listed is available for immediate sign up.
I'm pretty sure you can.
You might want to read some more about the tours to see if they will suit you:
https://www.ricksteves.com/travel-help/tours-faq
Assuming you mean the RS tours, the tour does not include airfare, and starts in Palermo. So you have two get there on your own anyway. Its recommended you come at least a day early to deal tic jet lag, and to get settled before tour starts.