“There’s enough to eliminate anyone who gets too curious…”
Since you appreciate the friendship with the brother-in-law and for this reason are choosing to hang out with the other people for a little while, I think the best thing to do would be to roll with it, keep it smooth. It’s all sticks and stones, just say “things are going pretty well, thanks for asking”.
“What, all of it? The investments in Bermuda, Luxembourg, and the Cayman Islands, the account in Switzerland? I’ve paid my 14% tax but I have a little left. Why do you ask? Do you want to see my tax returns?”
My mother says to always put it back on them and say, “Why do you ask?” Or you could say, “You go first!” Or use a scary, low voice with an accent and reply, “I’ll never tell!”
“I appreciate your concern and I think it’s really wonderful that you want to make sure your brother’s family is taken care of, but you really don’t have to worry about me.”
I bet she won’t correct your charitable assumptions.
It would upset me enough to be ask that question once, but to be asked repeatedly would turn me into a lunatic, and would probably respond with something like: “Why on earth do you think it’
s any of your damn business what I’m worth, aren’t you embarassed by asking such a personal question” or you can go with a give me your ear motion and whisper into her ear in a low voice. “None of your fucking business, stop asking”
I know it’s important to keep the peace, but there’s limits this would be one of them for me.
Winner, winner, chicken dinner.
<Doctor Evil Pinkie Gesture> “One MILLION dollars.”
The classic response is ‘nunya.’ When she indicates that she doesn’t know what nunya means you reply, ‘Nunya damn business!’
The lunatic response (which works best if you’re sitting at a table) is as follows: Look her straight in the eye and flip that goddamn table without ever breaking eye contact. Then calmly sit back down.
That’s good. ‘Enough to keep the cops on the wrong track for another decade,’ or ‘I never need to use the same murder weapon twice,’ would also work.
I agree with this one. Otherwise, go the “Why do you ask?” route. (If she has the gall to say she thinks she’s entitled to some of it, chuckle gently, pat her hand, and say, “Oh, you! But seriously, my will leaves it all to” whatever your favorite charity is. “I know you’d never want to deprive those needy (whoever or whatever) of the financial support they so desperately need. My husband always said what a kind, generous woman you were.” Of course, you can lie about that if you want.)
“According to my accountants, enough to make my estate donation to <Charity that Opposes Everything They Stand For> very, very happy.”
Or, “About 4.50, according to the element breakdown. I round up to 5 though; my liver is FULL of copper!”.
I’d say “Sorry but I’m not telling you that. We are fine.” And out of curiosity maybe follow up with “Do you get an answer from other people when you ask that question?”
“Not your business. (An offensive, impertinent question. - optional) Do not ask me again.”
Said calmly, while looking straight into their eyes. Turn on your heels and walk away to another spot.
Done. And done!
Good one!
I was thinking about that, too - how much the human body is worth when broken down into its parts.
I’d go with the humourous answers if you want to keep seeing her every year or so, and don’t want to tell her to take a flying leap (even though her bare-assed audacity at even asking is breath-taking). Make a game of it - see if you can give her a different flip answer every time she asks. 
ETA: Forgot about the cross on the forehead thing - that really needs to stop. How about telling her, with a big smile on your face, that when she lifts up her arm to do that, she leaves her whole stomach exposed for a knifing or a punch or something?
Take out a voodoo doll and stab it frantically with one hand while scrubbing your forehead with the other? Or pull out a sharpie and draw a pentagram on her forehead.
You could tickle her mercilessly whenever she tries to put the cross on your forehead.
Or you could do an upside-down cross on hers at the same time she’s doing it to you. Be sure she knows it’s upside-down.
Or color her nose with a red marker while telling her she’s out of line. 
Ask her if she swallows or spits.
Scream at the top of your lungs, then say, “I’m so sorry, you startled me.”