Pay down the $800 to zero. Don't take on any more debt.
Do use your credit card and pay it in full each month and on time.
Continue to pay your car payment on time each month. If it will save you considerable interest, try to get it paid down, but, saving for a down payment is probably a better strategy.
Good luck! My husband and I bought our first house at age 21-22.
You should arrange some kind of rent agreement with your sister, to show that you are making regular housing payments. Don't pay in cash and keep records.