Again with the poop?

There is something going on with this kid... mind you, Sophia has been completely potty-trained for well over two years and has only had 2 accidents since then, until recently. I walk into our bathroom downstairs to wash my hands, right into an overwhelming stench of poop. I look to the garbage can and see a garbage bag full of diapers. As my mom is in town watching the kids this week (we have no daycare just this week), I look at the bag and think "Mom must have left one of Noah's poopy...wait... what is that??" I look in the clear, plastic garbage can to see a large turd lying in the bottom.
  "Sophia, come in here!"
  "Yes, Mommy?"
  "Did you poop in the garbage can?"
  "Yes Mommy?"
  "Why???"
  "I had to go potty." (matter of factly)
  "The potty is RIGHT THERE!" (the toilet is literally 6 inches from the garbage can)
 I proceeded to give her some clorox wipes and made her clean out the garbage can. I also learned today that Brandon had to clean out the garbage can in her room because it smelled like urine, AND that she pooped in the tub the night before. Something's really going on with this girl, now to try to discover what it is. Noah won't even look at the potty- let alone use it! Correction... the other day, he DID want to try to go, but once I grabbed his seat to put on the toilet he yelled "No, tand up!" So I grabbed the step stool and helped him climb up. He stands there, his penis pointing straight at the floor. I tried to get him to aim into the toilet, but he was having none of it, so what do I do? I spent the next two minutes aiming his "peenie" into the toilet as my 2-year-old grunted and tried to pee into the potty, to no avail. Well, at least he tried, right? What a zoo!