what would you do?

ok my sister and her bf of 6 yrs have a son who`s 18 mos old, they both work and when they get off work it seems as if they are to tired to care where there son is at, i live next door to them, they let him walk to my house leave him outside by him self, and one time when i was driving to go see my mom and dad they took him out of the car seat because he was crying and needed a diaper change, so i pulled of on to an exit so they could change him and they were like why you stop we do this all the time. im scared for him and my bf says the same as i do its not right how they are with him. my friend does the same thing with her kids and if i say anything to my sister or my friend they say stop being so worried, its my kid ect also my sisters bfs mom and dad watch there son everyday and they to drive around with him out of the car seat. it pisses me off.