Babysitting while in law parties

How can I tell my husband that I don't want to babysit his niece and nephew while his 43 year old sister goes clubbing? I'm 9 weeks pregnant and a mom to a 10 year old and a 6 year old I feel drained my 2 girls are a hand full already and his niece is 9 years old and his nephew is 2 years old. I find it very unfair to have to work all week even Friday and Saturday and have to come home to babysit while his sister goes clubbing and drinking. Last time we babysat them his sister said that she would pass by and pick them up around 2:30 am I woke up at 3 thinking we had not heard her knocking so I tell my husband to call her and see if she was on her way and he did and she told him that they were "to tired" to pass by and pick them up. The next morning she didn't come until 11 am. I just want to enjoy my only day off without the stress, also my 10 year old  daughter and his niece do not get along at all, all they do is fight. I want to tell him, but don't want to cause any arguments. Advise please?