And then comes the collimation. It seems that especially newtonians need collimating - means aligning the mirrors.
This task is said to be easy after you have done it a few times. You shouldn't need to do it every time...maybe.
I hate working in the dark so I sold my newt and got a refractor 120mm - although I haven't used it yet.
It would probably be helpful to seek out an astronomy club and attend a viewing night, with you sister. The people will gladly show off their equipment and you'll see what size you like.
This site has great info on what you will see in a 6" scope anytype:
http://www.geocities.com/the_150mm_reflector/
I used it a lot when I was deciding.