Sicily in June.... how much might I wear long pants?

Of course you can as it will be very hot by then.I doubt you will want to wear them too often other than at night for dinner.

Probably. It depends on where you will be. If you are going to the top of Mt. Etna, or even just the lower wine areas like Randazzo, you may find you need a second pair. I'm seeing mid 60s at night in that area. Weather every where is just hard to predict these days. We were in Sicily mid April thru mid May, last year, and half the time we were cold (and we are from Minnesota). We had planned for warm, even hot weather, and we had perhaps 3 out of the 24 days, I'd consider hot.

Still, I tend to pack very light, I think you could make due and wash out the pants or buy a second pair if really necessary.

I’m glad that you are not sharing the same pair of pants with your wife and you will have one pair each.
You will not need them during the day unless you are going to high altitudes (like Mt. Etna). In the evenings it might get cool near the coast, especially after a rainy day (which is less frequent in Summer but possible), but you can probably survive without pants even in the evenings. How about adding one extra pair to be shared with your wife, so you will have 1.5 each?

We went to Sicily in late May and early June, and were miserably cold on Mt Etna. Now, I'm not cold tolerant at all, but it wasn't just me. The wind was swirling, and it was snowing! Almost everyone decided the snack bar was a much better option than walking around outside.

What about a pair of "travel pants"- so you could zip off the lower legs and have shorts when needed?

My husband has always brought two pair of pants even when we went to Italy in August because the alternative is shorts which are not appropriate everywhere. I, however, have not. I usually bring one pair of pants, a skirt and capris which are cooler and more flexible than pants. The only time this was not adequate was when we hit uncharacteristically cold weather this May in Sicily (think 50s and driving cold rain).

Mt Etna is chilly but that is usually only one day so no need for multiple pairs of pants just because of that.

We use long pants to travel on the plane to get to our destination, never shorts, so we have one pair of long pants with us.

And in response to the poster that recommended "zip off" pants, in my mind, those are the tackiest items ever created. Please don't wear them, unless you are on safari in Africa. They are not 'travel' pants, they are for hiking at best.

So, you don't need to pack long pants if you are smartly wearing them from home on the plane, no need to pack an extra pair in summer in Sicily.

My husband has several pairs of zip off pants. I absolutely hate them, when used for anything other than camping/hiking! However, he "runs" hot. He could almost wear shorts year round. He always brings a pair to Europe because the thought of wearing pants all day in Italy to visit churches, etc. was too much for him. I will admit, they do work well for him.