Babysitting

My dear sister, who's also my roommate, agreed to babysit my cousin's two girls , 5 and 8 years old.
I had no idea of the arrangement. Well, I get home from work and here they were. The following morning, my dear sis took off to do her things, and left me with the girls. 9, 10, 11, 12 o'clock came and went and no sign of my sis! I wound up feeding them breakfast, lunch and bathing them, dressing and combing their hair. And. I had to cancel plans I made for lunch with my friends because my dear sis wouldn't answer her cel.
So I asked my cousin to please, out of courtesty to me,the next time she and my dear sis made plans, to text me so I would know, and that way I could make/ rearrange plans that I made. My very "considerate" sister never told me about any plans and never will.
My equally considerate cousin refused saying it was "our" problem and we needed to work it out our selves ! I so don't agree.