I'm trying to come up with a list of films and episodes with a similar premise to this- sister ending up looking after children. It's not going so well- so far I have this, Baby Boom and the episode of Friends with Rachel's sister. Would really appreciate any other ideas!
Thanks!
No Reservations is the first one that comes to mind, love that movie (Abigail Breslin also plays the little girl in that). And its not about a sister but Life as We Know it with Katherine Heigl and Josh Duhamel is about two people having to take care of their mutual friends baby.
And it hasn't been released yet, I've only seen the trailer, but Gifted looks to be similar, except its the brother taking care of the kid instead of a sister.