One of the hardest parts of having an addict in the family is how it complicates relationships within the family. It will take some time before your sister figures out what her boundaries are and what yours are with regard to your cousin.
My sister used to take my AD in when she was not welcome at home. In some ways I was glad because my AD was somewhere safe where she could cool off. In other ways I worried because I knew my sister might be victimized by having my D in her home. So, I always made sure that my sis knew the score with my D and let my sis make up her own mind about how she would handle situations. Over time, my D pulled enough stunts on my sister that my sis had to accept that she could not help my D. However, I know that my sis had to come to this on her own terms, in her own time.