If your sister goes with your brother then it's incest and they would both be your siblings. If a girl is dating your brother and they get married this would make her your sister-in-law.
Your brother or sister can never become your brother-in-law or sister-in-law. The people they marry are your brother-in-law or sister-in-law.Oh, yes, in a riddle, if your brother marries your sister,...
I believe it would be similar to mom and dad. If you said, my Mom went to the shopping center, you are using the word mom as her name, but if you're referring to who she is... as being a mother, I...