NEED ADVICE FOR MY IN LAWS

my in laws rely on me everyday to take their son and pick him up from school. At first I didn’t mind because I’d pick him up maybe once a week and get paid $10 for it and they and their daughter (my sister in law) took and picked him up the other days. It slowly became me being the one who took him to school and since I worked graveyard shift I wasn’t too happy with either not being able to stay overtime for extra money or the fact that when I got home I had to stay awake to take him to school. For some reason they enrolled him in a school more then 5miles away! So that was for 2 years. When this school year started my father in law asked if I’d take him to the bus stop (literally down the street) for just the first month, I said okay because at this time I left work to be a full time student but I was home. A whole school year later in the only one TAKING AND PICKING HIM UP. They will not even TRY to do this. When I have appointments (I’m disabled and now pregnant) they say “well can you make back home to pick him up) and when I ask for them to do it they give me major attitude! This is their child not mine! Also this kid is 13 why can’t he walk less than a mile to the bus stop?! HOW CAN I TELL THEM THIS IS TOO MUCH FOR ME AND THAT THEY NEED TO HELP?