Sounds like the wax seal at the base of the toilet, where it bolts to the floor, is leaking! If you are handy with tools, here's what to do: go to Home Depot or Lowes. or the local neighborhood hardware store, and buy the wax ring for the seal to the floor! On the base of the toilet tank should be a water shut-off valve! If not, go down cellar to where the main water supply line is, and shut it off! Go back up to bathroom, flush the toilet. pry off the bolt caps, remove the nuts, lift up the toilet from the bolts, pry off the old wax seal, clean the area well, lay in the new wax seal, reinstall the toilet, install the nuts, don't tighten them so much that you crack the toilet base, install the nut covers, connect everything up, turn on the water, and you should be good to GO! and, you'll really impress the MIL by your prowess!
Some applications of Drain cleaner should fix the water drain back-ups! Drano, or some other hi-test cleaner, like Liquid Plumber should do it!
The only other reason for drain back-ups is: you are overloading yer septic tank and system, what with the dish-washer, and many loads of wash! The drain fields are probably plugged up, which can cause many problems, especially if your septic tank and system isn't big enough to handle all of the water you are putting into it!
Try this: do the dishes by hand for a while, and take some or all of the laundry to a laundry place where they have the machines, or cut down on the number of days you do the laundry, so you can see if that helps the situation! Another thing--don't flush the toilet every time somebody tinkles in it! (That is what some of the rural folks have or had to do with being on a septic system! I know, its not how you were brought up, but its necessary at times when on septic systems!) Do this, until you decide if that might be helping the problem, too! You might need a bigger system installed, like larger tank-s! Get the plumber to do a dye test on the tank-s and fields and laterals.
Good luck: rustyj