Grossly oversimplifying, there are basically two ways to get rain in the summer Sierra: the rare Pacific low-pressure system from the west and the more common monsoonal storm from the east (coming in backdoor from the desert Southwest).
The latter is what we have right now. I'd think by the weekend this pattern should be cleared out, so at worst you're looking at the chance of afternoon showers, not all-day rain they're probably getting now. I wouldn't cancel but I would keep an eye on the forecast as you get closer and have a west-side alternative if you don't want your newbies to have a soggy trip.
Last August we had to take an unplanned layover day while it stormed, because I didn't want to do our XC alpine pass in a thunderstorm. It happens. Have also met a party coming out of the John Muir Wilderness on the east side who had endured rain every day of a weeklong trip, while we never saw a drop during our week. Just last June we were chased out of the Tahoe area a day early when a cold storm hit with horizontal rain and temps in the 40s. Yuck.
I have not dealt with Sierra thunderstorms before, only rain and mild snow. It says 20% chance so I presume fairly unlikely - are there any documents you recommend reading that can provide some general safety tips if one does come? Honestly if we see a thunderstorm coming we will probably just head back to the car, which will be 6-7 hours away.
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