The tank of the upstairs toilet won't refill until we turn on the bath or sink faucet. Any idea what is wrong or how to fix it?

Now, that is indeed odd. I guess the simplest, cheapest place to start would be replacing the fill valve in the toilet tank. If that doesn't cure the problem, please come back and we will try to figure this thing out with you.

Now, that is indeed odd.

Agree.

I'm going to go out on a limb and say that there is something blocking the stop valve. I'm going to go further out on the limb and say that the thing creating the obstruction is a piece of solder that has broken loose and moved to the stop valve.

Careful, Harold, that limb gets mighty unpredictable the further you go out. Haaaaa. I actually wish I could see this first hand. I kind of enjoy new experiences (more-so when I can figure out a quick fix), kind of makes things interesting.

I actually wish I could see this first hand.

Me too.

Maybe you can see it from the limb you are on. Im sure there must be a good view from there. Hee heee.

Good one.

I'll let you know on the way down.

Just wish I were there to catch you.