Am I missing something?

I grew up going to church and Sunday school from infancy until I graduated high school.  Sis and I were episcopal until about 4th grade, then started going to a Methodist Church til high school, then catholic services.  I consider myself of the Methodist faith because I agree with more of their beliefs than other.  Many many years ago my sister was visited by a couple of young men (not Jehovah) who talked to her of their faith and so she switched.  No big deal to me.  It's not my type of religion but she's happy.  Well a couple months ago my niece was baptized and my sister said that ever since then she's become (don't know what to say about it, but the examples are)...before my niece was baptized they used to go over her high school friend and daughters house on Sundays, watch movies and sometimes sleep over or go to a movie and out to eat.  Now since baptism my niece dictates that nobody in the family can go out because its Sabbath.  Sis says if they are talking about doing something niece will hold out her hand and say, nope, nope we can't go out, it's Sabbath and so they don't.  She said her friend has become kind of annoyed and my sister tells her, well I can't ask her to go against her beliefs.

So, what I want to know is am I missing something.  I don't remember ever learning you couldn't go out on a Sunday.  Sunday was a day of rest but we'd go over to family or go to get something to eat or even a movie. 

I mean it's their life and if that's how they want to live it's their life, but now they are letting their 8-year-old decide what they are going to do (it sounds like - I don't live near them).  Sis says my niece is telling everyone about the Bible and God and preaches to them and just says things as though she knows more than others.  My sisters proud of it and encourages it.  I'm finding I'm getting creeped out about it.  She's an 8 year old girl.  I think it's important to learn about Jesus and his teachings, but I just don't remember any of what they are saying.

Can someone tell me where it says you are not supposed to go out on Sundays.  Like I say everyone has their own take on religion, but there is something about this that is just kind of making me wonder what in the world are they teaching this girl.