as it is almost xmas you could do the brotherly love thing and ask her to limit how she views your movies,
you could for instance ask her to limit streaming by copying the movie from the homegroup to her hardrive, then she can delete it once viewed.ask her to copy one or two a day as she probably doesnt watch more than that.
explain that streaming eats up the bandwidth which is no good to anyone else who is sharing it, failing that get your mum and yourself to leave the homegroup set up a new one with a new password, and dont tell her the password.
or if its possible change the password daily letting your mum have it and your sister has to do your chores before you give her the password.
but remember sisters can be evil when it comes to payback.
and it is nearly xmas , remember sharing is caring!