If you’ve decided to make the word sit one of your commands, then everyone that talks to the dog needs to use the same exact command spoken in nearly the same tone, and not use sit down, sit up, sit over there or other confusing sentences.
It is also important to always act immediately anytime there’s some kind of accident, such as urinating in the house. Finding the urine and an hour later is not going to be a training exercise that will be worth your dog’s time. Catching it immediately is the only way it works.
However, since most dogs need to urinate and defecate almost immediately when starting on a walk, the absolute best way to train them is to take them outside and have them run or walk until they’ve finished their business. Praise them profusely and give them a treat so they know they’ve done the right thing at the right time, and that is one of the secrets to training a dog right.