It's awful to have to make that decision about a pet, but you guys did absolutely the right thing. *hugs* She got to know what it was like to have people who cared about her in the last weeks of her life, and that's a good thing, even though it makes things sad now.
I would take your other kitty to the vet and get him checked out. That way, if he's healthy, you know and don't have that on the back of your mind. And if he's not, then you can love on him and appreciate him extra special while you have him and he's healthy.