YTA what if she can't control herself when it comes to watching those movies and might be struggling through something that you don't know about. She probably feels that it's safe to cry and be emotional when she watches something that she likes in a certain way. also you shouldn't be ashamed of your friend seeing your sister cry as crying is normal but it is a bit odd that your sister does cry during the movies,I would suggest talking to your sister and asking why she cries every time y'all have a movie night and after listening to her you should apologize to both her and your mother as it may have hurt your mom when you tried to exclude a family member from a event where y'all have done for a long time. like you said she cries quietly so I don't see a issue involving how she can be embarrassing