Well first of all, you need to tell your mother-in-law to back the heck off. Or better yet, have your husband tell her.
Secondly, try not to sweat things too much. I managed to potty-train both of my kids without a single book or chart or anything else. Not that those things don't help, but I'm the type of person that tends to overload myself with information. Which is fine in most things, but potty training is NOT one of them. Honestly, I'm the most laid-back person you will ever meet and I think that approach worked well for my kids when it came to training.
As far as specific advice, I would say three words: BRIBE, BRIBE, BRIBE!!! Whenever my kids would manage to go on the toilet, we'd grab them up (after wiping, of course) and dance around the house with them singing "Pee-pee on the pot-ty!" or "Poo-poo on the pot-ty!" (to the tune of a conga line) like they had just won the jackpot on a gameshow. Then we'd give them an Oreo or some other indulgance while bragging loudly to each other about what a big boy/girl they were. The first few times, we even had them call the grandparents to share the good news. Basically, we took "positive reinforcement" to the extreme.
We did this pretty consistently for a few weeks, then started to taper off when the kids got better at taking the initiative and going on their own. And they were so proud of themselves, they didn't even notice the lack of bribe.
Another tip for when your daughter goes poop in her diapers is to have her go into the bathroom afterwards and (carefully) remove her diaper. Then have her watch while you dump the poop into the toilet, telling her that's where it "belongs." And let her flush after, kids love that. It worked well for me, anyway, and after a while they caught on.
And I know it's a little weird, but having your daughter watch while you go once in a while is a fantastic training tool. And if you have a boy, your hubby can help with him when the time comes.
But really, like most of the ladies have told you, one day it will simply click for your daughter. And any agonizing you do now over how old she is and whether or not you're doing it right will count for exactly SQUAT at the end of the day. So give it time and give yourself a break, huh? You deserve it.
Good luck!