Boyfriends family

My boyfriend and his family are Belizean & his mother and grandmother aren't the nicest. They're way too controlling and he's 25 they expect him to take care of his 20 year old brother & his 11 year old sister. We've been together for over a year and we just moved into our first apartment together. This is the first time I moved out from my parents so I'm super excited. We've only been here for 1 week and 3 days & His mother asked him already to baby sit his 11 year old sister EVERY Tuesday from 5pm-8pm. I know it's not a long time but WE JUST MOVED IN & she's a bitch sometimes so I feel that I'm not obligated to help her at all! Anyways my boyfriend wants to have her come to our apartment to take care of her. I feel bad I said no because we just got here and she should get a baby sitter. His mom told him he better do it because she helped him pay for school & she gave birth to him so he has to help. Am I wrong for saying no? I told him that he can go to his mother's house and sit with her until his mom came home & that he didn't need to be at Our apartment... he lives only 2 miles away from our new apartment. What should I do?