I'm going to tell my sister it's for the best
What is for the best for your sister?
I want to relieve some stress
It is normal to want to relieve stress and it is good that you want help from your sister. It is good to help each other as family members.
I want to tell her how much she sucks and how useless she is.
Perhaps your sister needs a wake up call since no one will tell her the truth, but go about it in another way. Confrontation and harsh words won't make her feel better or change, show her how she could be doing more and how you wish she was trying.