Dream of saving my sister??

I have a sister who is 5 years older than me and lives in a different country, I've found out she was doing bad things, but before the things she did occured she had a dream, there were black scary dogs attacking her and there was a black door, then I was there to save her. What does this dream mean? I don't know if this is just some meaningless dream, what do you think?

(Sorry for my English, it isn't my first language)