Regicide
A murderer or perhaps a Kinslayer. The term you are probably looking for is "fratricide"
She will be referred to as the sister of someone's brother-in-law.
Your brother or sister.
a butcher
Simply your brother or your sister. My best friend is of a difference ethnicity and family, but she's my sister. Family means someone you love, someone who helps you through thick and thin....by blood or not.
Rene (the guy who kills his sister)
Fratricide is the killing of ones brother or sister So you would call him a( Murderer )
Yes, calling someone "big sister" can show respect, admiration, and a close bond between siblings. It can be a warm and affectionate way of addressing an older sister.
A person who kills plants is generally referred to as a "plant killer" or someone with a "black thumb."
A gatecrasher.
Your brother's wife would be your sister-in-law. If you are introducing her to someone you can say (example) 'This is my sister-in-law Jane,' but otherwise, simply call her Jane.