Hello!
I think I had the same problem . I have a brother and he was like your sister I think...well not that bad now but about 1-2 years ago it was..
He was so stupid, he didn't have job, he came home late night everyday and then he was sleeping all day long. When we tried to tell him that's not good he got mad and started arguing. He could ruin our mood in a minute..and then of course my parents got sad and mad too....and I couldn't do anything
But it's ok now..my brother has job, and he's quite normal now
I hope that everything will be allright in your family and with your sister:)
Cheer up!:kiss: