I have two words for y'all: Cousin. June.
The Queen of One-Upmanship. OK, that was more than two words - and I will say more.
My advice to Tammy: ignore her, be neutral about everything she does ("oh. That's nice. Listen, I have to go wash my hair/wax my thighs/stand over there *points* now.") Ignore, ignore, ignore.
Let her know what is clearly out of bounds - "I'm sorry, but those papers aren't for you to look at. You should leave now, please" - and show her the door. As to the kids' grades, "I'm sorry, but we've made it policy not to disclose the kids' grades - we're just happy they did well." Make sure the kids know about the policy, too - not to drag the kids into it, but she did start it by nosing around with the kids and hubby. Every time she comes sniffing for information, throw cayenne pepper in her trail (figuratively speaking, of course). Put her off.
Refuse to (directly) discuss any decisions of ANYTHING you are going to do, vacation or activity-wise. "Oh, I can't decide if we're going to Disneyland Paris or Australia this summer, we just haven't figured it out yet. [and your cabin at Yosemite has been reserved for months already]" "Oh, I can't decide if I want to take underwater basket-weaving or if I'm going to take up the sushi-preparation class." [and you are already signed up for Aikido] "Hmmm, we still haven't decided if we want the Jag or the Lexus." [and you've already gone to the dealer's and signed the papers on the Acura] And so on.
The key here is Cheryl not getting to Tammy. My cousin June invited everyone over for Christmas Eve dinner to admire her house, her things, but especially her new, custom-built, backyard swimming pool (who wants to admire, or use, a pool in DECEMBER? It's cold outside, even here!). My mom (who was there; I was home sick with the Killer Kold from Outer Space, which I still have icky remnants of), remained neutral about everything. "Oh. You have a pool. How nice."
"How nice" are my two favorite words in the English language that substitute for "yawn." I mean, honestly, I grew up in California (true, it was the Northern part, but summers here can get pretty hot inland, and pretty much every other house down South has [surprise!] a pool in the backyard) and I went to college in SoCal - how many backyard pools haven't I seen in my life?
The last time I saw her that she had something to try to "impress" me with was her new Toyota Sequoia (I live in suburbia - SUVs and minivans aren't anything out of the ordinary to me). My reaction? "Oh, a new car. Call me when you take delivery on the CitroŽn." And I walked off.
Evil, but it worked. She doesn't know what a CitroŽn is, or why I would be more impressed by that than by your everyday, gas-guzzling, atmosphere-choking, parking-lot-crowding SUV.